Structure and photophysical properties of furoquinoline aza-helicenes: Fluorescence enhancement by protonation

Abstract

In recent years, the introduction of one or more heteroaromatic rings into a polyaromatic backbone has attracted growing interest as it allows significant control over optical, electronic and supramolecular characteristics. Polyheteroaromatic compounds are potential optoelectronic conjugate materials due to their photo- and electrochemical properties. This work describes the structure and photophysical properties of the first representatives of azaoxahelicenes of the furoquinoline series, synthesized from readily available starting compounds. Using X-ray diffraction analysis and DFT calculations, it was established that furan-fused aza [5]helicenes, unlike their carbon analogues, do not undergo Mallory-type photocyclization due to an increase in the distance between the terminal carbon atoms, which determines their greater resistance to UV light. A study of the acidochromic properties of the obtained compounds showed that protonation with trifluoroacetic acid (TFA) leads to an increase in the fluorescence intensity of most fluorophores. The best fluorescence enhancement was obtained for a fluorophore containing a methoxy substituent in the peripheral phenyl moiety, with a 10-fold increase in quantum yield. The introduction of a furoquinoline residue into the helicene skeleton leads to an improvement in fluorescent properties, as well as the possibility of their modulation by acidic agents. In the future, the first representatives of azaoxahelicenes of the naphthofuroquinoline series will be used to design new materials for optoelectronics and study their performance properties.