Electrochemistry of Molecules for Singlet Fission
Authors: Ludvik Jiri , Šimková Ludmila , Lušpai Karol , Klíma Jiří , Liška Alan 
Published: September 2023

To improve the maximal theoretical efficiency of relatively inexpensive single-junction solar cells, a promising principle of Singlet fission (SF) can be used. SF is a photophysical process in which an organic chromophore absorbs a photon and in excited singlet state shares its excitation energy with a neighbouring ground-state chromophore and both are converted into two triplet states which after charge separation offer theoretically up to two lectrons.1,2 Although up to now, it has not found its way into practice, the synthetic, photophysical, electrochemical and computational research is running.
SF is possible to describe as an exchange of electrons between two molecules which does not require change of spin of any from the molecules. The most important criterion in the selection of chromophores, assures that the SF process is isoergic or even better, somewhat exoergic, by demanding that the first singlet excitation energy be at least the double of the first triplet excitation energy. This criterion is quite unusual. A class of compounds that are likely to meet the requirement are biradicaloids, species formally derived from perfect biradicals by sufficient but not excessive covalent perturbation 3.
When a promising molecule is designed, synthesized and tested, several fundamental properties have to be met. Very crucial is their crystalline form and molecular packing, nevertheless, the energetic criteria are the most important: (i) the relative energies of the S0 (lowest singlet), T1 (lowest triplet), and S1 (first excited singlet) states, and (ii) the potentials of one-electron reduction and one-electron oxidation. Redox properties of molecules for SF are thus critical for their use, therefore electrochemistry is a necessary approach to the investigation of promising compounds.
In our contribution, three types of molecules were selected and electrochemically tested in aprotic media and inert atmosphere: 1,3-diphenyl-isobenzofuran derivatives (DPIBF)4,5, molecules derived from cibalackrot (indigo family)6, and 2,6- (2,5)-diketopiperazines7. From the electrochemical point of view, very important is the reversibility of the redox/excitation steps, which are directly connected with the final stability and durability of the device. The difference of potentials between the first reversible oxidation and reduction steps reflects the HOMO-LUMO gap of the molecule. Very important are also the steric barriers in covalent dimers preventing fast recombination of spins.
For electrochemical investigations, the classic electrochemical techniques (polarography, CV and RDE) were used and combined with in situ UV-vis and EPR spectroscopy. The experimental work was accompanied by quantum chemical calculations.

Acknowledgement The authors thank to dr. Jiří Kaleta and dr. Miroslav Dudič for granting the compounds and to prof. Michl for valuable discussion. The grant 19-22806S (GAČR) and the institutional support RVO 61388955 is acknowledged for funding.

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