Synthesis of monodisperse aromatic azo oligomers toward gaining new insight into the isomerization of π-conjugated azo systems

Abstract

π-Conjugated aromatic azo main-chain polymers are commonly distinguished as promising photosensitive materials. However their rigid-rod structure decreases their photoswitching efficiency. The reasons are partly considered to be enhanced π-electron delocalization, London dispersion forces and interchain hydrogen bonding interactions. Herein we precisely synthesized monodisperse fluorene-azo oligomers (FAZO1, FAZO2, FAZO3 and FAZO4) with steeply extended π-conjugation lengths, and found that their photoisomerizations can be driven by visible light. The photoisomerization efficiency and rate decreased progressively with the π-conjugation length, which was well proven by theory. DFT computation revealed that only one end azo unit undergoes E → Z photoisomerization from FAZO1 to FAZO6 under irradiation, demonstrating a trans-[trans–trans]_n-cis model. The current results provide another basic guide for designing visible-light-driven photosensitive materials from π-conjugated aromatic azo main-chain oligomers/polymers.