π-Expanded coumarins: synthesis, optical properties and applications

Abstract

Coumarins fused with other aromatic units have recently emerged as a hot topic of research. Their synthesis is partly based on classical methodologies such as Pechmann reaction or Knoevenagel condensation, but it also sparked the discovery of completely new pathways. In very recent years so-called vertically-expanded coumarins were synthesized, effectively expanding the portfolio of existing architectures. A subtle relationship exists between the structure of fused coumarins and their optical properties. Although absorption of UV-radiation and light is unifying theme among these π-expanded coumarins, the fluorescence properties strongly depend on the structure. The mode of fusion, the type of additional ring and the presence of electron-donating and electron-withdrawing substituents all influence the photophysical parameters. Recent advances made it possible to modulate their absorption from 300 nm to 550 nm, resulting in new coumarins emitting orange light. This review serves as a guide through both synthesis strategies and structure-property relationship nuances. Strong intramolecular charge-transfer character made it possible to reach suitable values of two-photon absorption crosssection. Photophysical advantages of π-expanded coumarins have been already utilized in fluorescence probes and two-photon excited fluorescence microscopy.

Synthetic Applications of Oxidative Aromatic Coupling—From Biphenols to Nanographenes

“…Coumarins are bright organic fluorophores with multiple applications in fluorescence imaging . Figure presents the structures of π‐expanded coumarins obtained by the Scholl reaction or by intramolecular oxidative coupling.…”

Helicity‐Dependent Regiodifferentiation in the Excited‐State Quenching and Chiroptical Properties of Inward/Outward Helical Coumarins

“…[10] We were spurred by our recent investigationw ithh elicitya sas teric force to control the phenomenon of photochromism [11a] and engineer amorphous property for development of OLEDs [11b] to investigate the extentt ow hich helicitym odifies excited-statep henomena and chiroptical properties. [12] The photophysical properties of coumarins continue to be as ubject of extensive investigations. [12] The photophysical properties of coumarins continue to be as ubject of extensive investigations.…”

The synthesis of lactone-bridged 1,3,5-triphenylbenzene derivatives as pi-expanded coumarin triskelions

“…However, biphenyl is not a rigid or planar molecule, with solution-phase dihedral angles measured to be between 30–40°, depending on the solvent. 3 Our group has been interested in bridging aryl-aryl bonds with lactone units, in the form of new pi-expanded coumarins, 4 to force rigidity and planarity in the aryl-aryl molecular structures. 5 Our previous efforts in this area have been to study the effects of restricting aryl-aryl bond rotation in various biphenyls 5a and C 2h symmetric p -terphenyl crankshaft-shaped systems 5b with one and two lactone bridges, respectively, resulting in new pi-expanded coumarin structures.…”