Iterative Synthesis of Dynamic Conjugated Oligomers Displaying Length-dependent Configuration Metastability, State-dependent Properties and Helix Emergence

Abstract

The emergence of structure and function at the mesoscale is critical for the design of complex molecular materials. In this work, we report an iterative liquid-phase construction of precision conjugated oligomers with alternating di- and tetra-arylsubstituted alkene (DAA and TAA) building blocks. Due to the dynamically reversible isomerization of TAA building blocks in the backbone, this system exhibited a complex library of constitutional stereoisomers with configurational metastability, as characterized by HRMS, chiral HPLC and 2D excitation-emission fluorescence mapping analysis. The dynamic oligomer system exhibits aggregation-induced absorption shifts and state-dependent fluorescence, and shows constitutional responses to thermal and light stimuli. During the self-assembly process, mesoscale helicity emerges in the P5 oligomer and evolves into regular helices (40-66 nm pitch) in the P9 oligomer, which can also be tuned by side group modulation. Experimental and computational studies suggest a dynamic transition of conjugated backbone conformations with increasing chain length from linear, rich in trans-alkenes, to twisted with alternating trans- and cis-alkenes, resulting in a stereoregular triangular chain segment for secondary structure generation and property emergence.