(Invited) Electrochemistry for Singlet Fission

Very recently, substantial attention has been paid to search for suitable chromophores for singlet fission. Singlet fission (SF) is a photophysical process in which an organic chromophore in excited singlet state shares its excitation energy with a neighbouring ground-state chromophore and both are converted into two triplet states. [1,2] In this way, one photon can generate theoretically two electrons which should increase the efficiency of inexpensive single-junction solar cells. This spin-allowed process is very fast (in ps or fs time scale), therefore much slower radiative decay is out of competition. Nevertheless, its detailed mechanism is still unknown. Although up to now, SF has not found its way into practice, the synthetic, photophysical, electrochemical and computational research is running. When looking for new molecules – candidates for efficient SF, the following properties are crucial: their crystalline form and molecular packing, the relative energies of the S0 (ground state), T1 (lowest triplet), and S1 (first excited singlet) states, where energy of the triplet state should be lower than a half of the first excited singlet energy, and the potentials of one-electron reduction and one-electron oxidation. Redox properties of molecules for SF are thus critical for their use, therefore electrochemical approach is necessary. Nowadays two main promising groups of chromophores are investigated for SF: 1) functionalized polyacenes, mostly pentacenes, and 2) organic molecules which are able to form biradicaloids, on which our research is aimed. Derivatives of 1,3-diphenyl-isobenzofuran derivatives (DPIBF) were chosen as a first group of molecules suitable for SF. Our study was focused on a series of fluorinated derivatives of DPIBF where the influence of number and position of fluorine atoms in the molecule on the redox potentials and mechanism is followed. With increasing number of F atoms, two discontinuous changes of behavior (mechanisms) were found [3]. The second family of molecules which are currently under our investigation are derivatives of a very stable dye cibalackrot which are based on indigo structure. Our current work is focused on the study of oxidation and reduction of a series of differently substituted compounds. In all cases, electrochemical investigation was performed using classic voltammetric techniques (polarography, CV and RDE) and their combination with in situ UV-vis and EPR spectroscopy. The experimental results are correlated with quantum chemical calculations. Acknowledgement The financial support of the grant 19-22806S (Czech Science Foundation - GAČR) and the institutional support RVO: 61388955 are highly appreciated. The authors are grateful to dr. J. Kaleta and dr. M. Dudic for granting the substances and to prof. J. Michl for valuable discussions. References [1] M. B. Smith, J. Michl. J. Chem. Rev. 110 (2010) 6891. [2] M. B. Smith, J. Michl. Annu. Rev. Phys. Chem. 64 (2013) 361. [3] J. Kaleta, L. Šimková, A. Liška, D. Bím, J. Madridejos, R. Pohl, L. Rulíšek, J. Michl, J. Ludvík, - Electrochimica Acta 321 (2019) 134659. Figure 1