Tetrathia[7]helicenes, high triplet energy systems: synthesis, photophysics and electrochemical characterization

Researches focused on the development of new organic materials, displaying high carrier mobility, strong optical absorptions and efficient luminescent emissions, are urging for optoelectronic applications such as OLEDs, non linear optics, OFETs, photodetectors and OPV. 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. Tetrathia[7]helicenes, [7]THs, are chiral polycyclic aromatic compounds with nonplanar screw-shaped skeletons formed by ortho-fused benzene and thiophene rings. To investigate their structure-property relationships, we have prepared and studied three new fluorinated [7]TH. 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. Low temperature photoluminescence studies reveal the green phosphorescence emission, being located at ca. 540 nm, of the helicene systems never observed before; the significantly high triplet energy emissions in the helicenes is quite unusual considering the size of the conjugated ?-system (featuring 7 ortho-condensed rings). 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.