Electrochemical Properties of Derivatives of 1,3-Diphenylisobenzofuran – Chromophores for Singlet Fission

Abstract

Nowadays, there are efforts to apply alternative sources of renewable energy. One of these sources is solar energy. Solar energy represents a renewable free source of energy that is sustainable and totally inexhaustible. The standard efficiency of solar cells is now below 30 %. Singlet fission has attracted rising attention because it promises to increase the maximum of theoretical efficiency of single-junction solar cells. While solar cells after absorption one photon generates one electron, singlet fission is a photophysical process, whereby a singlet exciton generated by irradiation splits into two triplet excitons [1,2]. In this way, one photon can generate theoretically two electrons which should increase the efficiency of inexpensive single-junction solar cells. Therefore, recently substantial attention has been paid to search for suitable chromophores for singlet fission.When looking for new chromophores for efficient SF, 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 are crucial. Redox properties of molecules for SF are thus critical for their use, therefore electrochemical approach is necessary.Molecules on the base of 1,3-diphenylisobenzofuran (DPIBF) attract an interest because of their possible efficiency for singlet fission. In the last few years we have started to investigate various types of derivatives of DPIBF. One of our first studies [3] 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. Next to that we studied series of dimer derivatives of DPIBF [4]. The location and the nature of the bridging unit between two DPIBF redox centres attracted our attention because in this way the shape of the molecule, its extent of electron delocalization and, thus, possible ability of formation of biradicaloid can be changed.The most interesting dimeric molecule appeared to be that one with the elongated π-conjugated system, the “quasi dimer” of DPIBF (2). Because of that, quasi dimer was investigated in more details in comparison with other three molecules by standard electrochemical techniques, the in-situ UV-vis and EPR spectroelectrochemical techniques. The three other derivatives for comparison were parent diphenylisobenzofuran (1), direct dimer (3) and methylene bridged dimer (4). The main goal of this deeper study was to characterize properly not only electrochemical properties of the molecules, but also their absorbance and fluorescence properties and stability in solvents. Acknowledgement This work was supported by the grant 21-23261S (Grant Agency of the Czech Republic) and by the institutional support RVO: 61388955.