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

Abstract

Structure-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 lowlying orbitals of the system could be the way to overcome the issue. Here we report a study on three fluorinefunctionalized 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.

Thia[n]helicenes with long persistent phosphorescence

“…The sulfur atom in thiophene has been shown to make a substantial contribution to the triplet spin density, and the low-lying n electrons of sulfur atoms play a crucial role in enhancing ISC. 26 Additionally, based on the research conducted by Hasobe et al , the ISC rate constant ( k ISC ) was found to be over 4 times larger than the IC rate constant ( k IC ) in the multiple sulfur-containing helicene. 57 Consequently, although the combined k ISC and k IC ( k nr = k ISC + k IC ) collectively determine the radiationless deactivation, the ISC process was speculated to be the dominant factor, suggesting efficient population of the triplet state for TB[ n ]H ( n = 4–8).…”

“…6,9,23 Additionally, organic phosphorescence devoid of metals can be achieved through strategic substitutions on helicenes. 24–26 For instance, Hariharan et al introduced bromine atoms to a planar perylene diimide (PDI), resulting in an octabrominated derivative. The steric effects of these substitutions induced significant out-of-plane distortion in the aromatic core of PDI, yielding a double[3]helicene.…”

“…Despite a synthesis by Bossi et al of fluorinated thiahelicenes exhibiting phosphorescence, they attributed this behavior to the inductive effect of fluorine rather than sulfur. 26 Typically, thiophene rings serve as building blocks for constructing thiahelicenes due to their excellent optoelectronic properties. 46 As a component of π-conjugation, sulfur (S) atoms are anticipated to facilitate n–π* transitions more readily, thus leading to a stronger SOC compared to thioethers.…”