Synthesis of methyl-substituted aza[7]helicene with enhanced configurational stability and chiroptical performance

Abstract

As an important building block, aza[7]helicene (A7H) plays a crucial role in constructing chiral conjugated molecules. However, the poor configurational stability of A7H makes it easily racemize, thus limiting its applications in chiroptical materials. In this work, we design and synthesize tetramethyl-substituted aza[7]helicene (TMA7H) to improve the configurational stability. Experimental results demonstrate that the racemization energy barrier (at 25 ℃) is significantly enhanced from 29.5 kcal mol−1 for A7H to 41.0 kcal mol−1 for TMA7H, indicating the higher configurational stability of enantiopure TMA7H which can keep its chiral configuration even at high temperatures. For instance, at 150 ℃, the racemization half-life of TMA7H is determined as 4.2 × 103 h (about six months), which is three orders of magnitude longer than that of A7H (1.1 h). Furthermore, TMA7H exhibits stronger circular dichroism responses and circularly polarized luminescence with higher dissymmetry factors, making it a superior building block for chiroptical materials.