SOMO-HOMO Inversion For chiral open-shell pi-conjUgated systeMs

Project Information

SHIFUMI

Grant agreement ID: 101041516

DOI

10.3030/101041516

EC signature date 14 February 2022

Start date 1 October 2022

End date 30 September 2027

Funded under

• European Research Council (ERC)

Total cost

€ 1 744 065,00

EU contribution

€ 1 744 065,00

Coordinated by

CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE CNRS
France

Periodic Reporting for period 1 - SHIFUMI (SOMO-HOMO Inversion For chiral open-shell pi-conjUgated systeMs)

Reporting period: 2022-10-01 to 2025-03-31

Summary of the context and overall objectives of the project

Designing organic materials combining several functionalities (luminescence, conductivity, magnetism) is of great interest for increasing the performance of opto-electronic devices used in displays (OLED), photovoltaics or even electronic conduction (transistors). In this field, the introduction of the property of chirality to π-conjugated systems has recently appeared as a new direction of research, resulting in new properties such as a specific interaction of the chiral material with so-called circularly polarized light or even a spin-polarized electron conduction (chirality induced spin selectivity, CISS). While extensive research is conducted on chiral closed-shell systems, few attentions have been devoted to their open-shell analogs, despite the added-value of spin property, attracting significant attention in a range of scientific domains such as display technologies, cryptography and spintronics.

The SHIFUMI project aims to design this type of organic material, more commonly called chiral radicals, and to study their novel properties of luminescence and electronic conduction. This objective will be achieved via a new conceptual approach, involving a control of electronic orbitals linked to fundamental aspects of chemical and configurational stability of chiral radicals.

This project will provide new insights on this specific class of organic materials which have not yet been explored, despite their strong potential for increasing the efficiency of display devices or even information processing and storage, suggesting new perspectives in "chiral" organic electronics.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

To date, my ERC project has been running for two years. The first two PhD students, Mr. M. Lebreton (ML) and Ms. A. Rysich (AR), began their doctoral studies shortly after the project’s launch, with the objective of exploring novel molecular designs of chiral organic radicals, corresponding to WP1 of the Description of Action (DA).
Following our recent work on SHI based on carbazole units, ML has further investigated this intriguing electronic configuration in organic radicals by designing SHI radicals where the energy levels of the HOMO and SOMO could be controlled without affecting the steric hindrance around the radical center. This study is ongoing, but all the target compounds have been successfully synthesized and characterized.
ML has also made efforts to design dimers of biscarbazole linked by acetylene bridges to investigate how SHI can stabilize the corresponding radical states. Such organic mixed-valence systems remain rare in the literature and are fundamentally important for studying charge delocalization along a chiral molecular wire.
AR's objective is to design new chiral materials by harnessing the synergy between chiroptical and magnetic properties. Stabilizing enantiopure chiral mono- and diradicals holds significant potential for advancing high-spin organic materials, with prospective applications in chiral sensing and chiroptical devices.
In her first year, AR focused on the key strategy of employing meta-connections within the molecular framework to stabilize the high-spin state of diradicals (WP1 in the DA). She is also working on an alternative strategy that aims to yield chiral diradicals, potentially incorporating SHI, with promising prospects for success.
A third PhD student, Kai Xiang Lai (KXL), joined the team last year and is currently focusing on designing organic radicals with SHI and luminescent properties, albeit following a different methodology than initially outlined in WP2. Recent studies utilizing the strategy described in WP2 to investigate chiral luminescent radicals have been published, but the resulting luminescence exhibited relatively weak polarization intensity. Based on these findings, we decided to explore an alternative strategy that we believe could yield more promising results.
Two postdocs also joined us last year: Dr. Martin Vuagnat (MV) and Maher Hojorat (MH). MV has been developing new molecular designs of chiral carbazole-based compounds, while MH is focusing on spectroscopies, particularly MCD and MCPL. They both stayed for only one year, and we are currently considering publishing their work.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Among the publications I put forward in the previous section, I would consider three of them as significant breakthroughs for the people working in this research topic. While the first one is directly connected to the ERC project, the findings obtained in two other studies were not really expected but planned to some extend since we previously obtained related preliminary results.
Organic radicals with inversion of SOMO and HOMO energies and potential applications in optoelectronics, S. Kasemthaveechok, L Abella, J Crassous, J. Autschbach,* L. Favereau*, Chem. Sci., 2022, 13, 9833-9847
In this contribution, we focused the discussion on SHI organic radicals and gave some important considerations regarding this particular electronic configuration in relation to the Aufbau principle, often claimed, albeit imprecisely, as being violated in recently reported publications. This review follows notably our pioneering investigation on the design of persistent organic chiral radical displaying an inversion in energy of the singly occupied molecular orbital (SOMO) and the highest doubly occupied molecular orbital (HOMO), namely SOMO-HOMO inversion (SHI, see J. Am. Chem. Soc., 2020, 142, 20409 / J. Am. Chem. Soc., 2022, 144, 7253). All these aspects appear particularly relevant for designing organic molecular materials with tailored spin properties, which are of high fundamental interest and may eventually offer new opportunities in organic spintronic and optoelectronic applications, both as semi-conductors, or as efficient radical emitters.
Interplay of excited-states on circularly polarized thermally activated delayed fluorescent chiral exciplexes, P. Sumsalee, R. Arun Kumar, F. Lucas, D. Tondelier, J. Crassous, G. Pieters,*, L. Favereau*, Adv. Opt. Mater., 2024, accepted
Chiral luminescent exciplexes are reported, displaying emission dissymmetry factors reaching up to 10⁻² in the solid state. Interestingly, our findings suggest that the locally-excited (LE) state of the chiral donor impacts the overall exciplex emission, particularly influencing the intensity of polarized emission. More than affording one of the highest intensities of CPL for molecular TADF emitters, these findings bring important aspects regarding the chiral exciplex approach and the parameters governing the underlying process at play in these luminescent bimolecular systems.
Negative solvatochromism and sign inversion of circularly polarized luminescence in chiral exciplexes as a function of solvent polarity, P. Sumsalee, P. Morgante, G. Pieters, J. Crassous, J. Autschbach,* L. Favereau*, J. Mater. Chem. C, 2023, 11, 8514–8523
The potential control of circularly polarized luminescence (CPL), especially its sign and switching at the molecular level without any chemical modification, is desirable, but remains a considerable challenge owing to the difficulty to finely control the magnitude and relative orientation of the associated electric and magnetic dipole transition moments. To address this challenge, we report the synthesis and chiroptical properties of innovative non-conjugated chiral donor-acceptor systems displaying CPL sign inversion as a function of solvent polarity.

toc.png