Tetrathia[7]helicenes, polyconjugated high triplet energy system: synthesis, photophysics and electrochemical characterizations

Organic polyconjugated systems (both small molecules and polymers) as well as polycyclic fused aromatic hydrocarbons represent a fundamental class of molecules whose electronic properties and solid state intermolecular interactions can be varied over a wide range, upon proper functionalization of the -scaffold. Therefore, the quest of new organic materials for specific optoelectronic applications (including OLEDs, OFETs, photodetectors and OPV) and displaying high carrier mobility, strong optical absorptions and efficient luminescence, are urging. Helicity coupled luminescent features is recently raising great interest in the scientific community for circularly polarized emission studies. In this perspective, tetrathia[7]helicenes, [7]THs, are chiral polycyclic aromatic compounds with nonplanar screw-shaped skeletons formed by ortho-fused benzene and thiophene rings displaying interesting luminescence emissions although not completely described. We have prepared and studied three new fluorinated [7]TH suited to investigate structure-property relationships in this molecular class. In the present communication we report on the synthesis of fluorinated rac-[7]THs; their photophysical and electrochemical properties are compared to the ones of a parent [7]TH. [7]THs display deep blue fluorescence emissions with 10% QY and subnanosecond lifetime. Low temperature photoluminescence studies reveal a green phosphorescence emission, being located at ca. 540 nm, barely observed before; this high energy triplet is quite unusual considering the size of the conjugated -system (featuring 7 ortho-condensed rings) and its shape strongly depend from the functional groups R1and R2. The fast radiative phosphorescence decay in the order of 100 ms, its bright intensity, as well as the small S1-T1 gap (ca. 6000-7000cm-1) are suggestive of a quite efficient spin orbit coupling mechanism affecting the excited state dynamics of these non planar conjugated molecules. Preliminary theoretical TD-DFT investigation will be presented as well.