Sparkling Organic Phosphorescence from Fluorinated Tetrathia[7]helicenes: Synthesis and Photophysical, Electrochemical and Computational Studies

Abstract

AbstractStructure‐property correlations in the thiahelicene family are often not trivial beacuse most of the functional groups present on the helical scaffold modify the conjugation size of the π‐system. Selecting fluorine‐containing groups to provide strong inductive effects without interacting with low‐lying orbitals of the system could be the way to overcome the issue. Here we report a study on three fluorine‐functionalized tetrathia[7]helicenes, highlighting interesting correlations between the position of the functional groups and the conjugated skeleton properties. Helicenes Heli‐F2 and Heli‐CF‐F2 were prepared by photoinduced isomerization‐electrocyclization (the Mallory photocyclization) of the corresponding fluorinated benzodithienyl‐ethenes Alk‐F2 and Alk‐CF‐F2, which were prepared in high yields through stereo‐conservative Stille reaction. Notably these helicenes were found to display green phosphorescence around 530–550 nm, and the studies suggest an efficient spin‐orbit coupling mechanism in these high‐energy triplet nonplanar conjugated molecules. Both helicenes and their precursors were thoroughly characterized by means of optical and electrochemical measurements, while DFT calculations enable a rationale on their structure‐property correlations to be defined.