Enantiomerization of five-membered-heterocycle-embedded helicenes: A DFT study

In this work, DFT theoretical calculations were employed to investigate the enantiomerization of helicenes embedded with five-membered heterocycles. The original benzene rings in the helicene backbone were replaced by heterocycles such as furan, thiophene, pyrrole, or phosphole to create [n]helicenes with n ranging from 4 to 7. The impact of the type, position, and number of heterocycles on the enantiomerization barrier was systematically evaluated. Notably, the enantiomerization barrier was found to be significantly dependent on the rotatory angle and the position of the heterocycles, particularly for [4, 5]helicenes. With less rotatory angle of heterocycle, the enantiomerization barrier of helicenes was revealed to be lower, while when the heterocycle was close to the central part of the helicene chain, the barrier was also lower. Furthermore, the number of thiophene rings also had a marked effect on enantiomerization, showing a decrease of the barrier with more thiophene rings placed on the helicenes backbone. We expect this work would deliver new perspective on the relative studies for the helicene conformational conversion.

Thia[n]helicenes with long persistent phosphorescence

Helicenes with persistent luminescence have received relatively little attention, despite their demonstrated highly efficient intersystem crossing (ISC) from the excited singlet to the triplet state. Herein, we designed a series of ortho-fused aromatics by combining dithieno[2,3-b:3',2'-d]thiophene (DTT) with annulated benzene fragments, denoted as TB[n]H (n = 3-8), to achieve persistent luminescence. Wherein, thia[n]helicenes (n = 5-8) exhibited intense phosphorescence with millisecond-range lifetimes (τp) at 77 K. Particularly interesting was the observation that the odd-numbered ring helicenes displayed longer τp values than their neighboring even-numbered counterparts. Notably, TB[7]H showcased the longest τp of 628 ms. This phenomenon can be attributed to the more favorable ISC channels and stronger spin-orbital coupling (SOC) of old-numbered helicenes than even-numbered ones. Furthermore, both conformers of TB[7]H exhibited significant circularly polarized phosphorescent (CPP) responses, with luminescence dissymmetry factors (glum) of 0.015 and -0.014. These discoveries suggest that thiahelicenes may be a specific class of organic phosphorescent and CPP materials.

Construction and Phosphorescence Behavior of S/Se-Heteroaromatics/Phenanthrene-Fused Hetero[9]helicenes

Three S/Se-hetero[9]helicenes were synthesized by an assembly strategy of two building blocks, phenanthrene and ternary fused S/Se-heteroaromatic ring, and characterized by single-crystal X-ray diffraction analysis. Their phosphorescence emission gave a long-lived lifetime at the millisecond level. In addition, the high configurational stability of enantiomer (+)-[9]BH-1a was observed without change under both the conditions of 300 °C in the solid state for 18 h and 200 °C in solution of 1,2-dichlorobenzene for 10 h.