Synthesis and optical properties of fluorene-azulene π-conjugated polymers

Abstract

The non-alternant aromatic hydrocarbon azulene as a potential building block for the creation of new polymer conjugated materials is attracting more and more attention due to its unique chemical structure, optical and physico-chemical properties. The azulene conjugated polymers were synthesized by the interaction of 1,3-dibromoazulene or 2,7-bis(3-bromoazulene1-yl)-9,9-dihexyl-9H-fluorene with 2,2’-(9,9-dihexylfluorene-2,7-diyl)bis(1,3,2-dioxaborolan) under the conditions of the Suzuki cross-coupling reaction: poly[2,7-(9,9-dihexylfluorenyl)- alt-(1ꞌ,3ꞌ-azulenyl)] 3 and poly[1,3-bis(9ꞌ,9ꞌ-dihexylfluorene-2ꞌ-yl)azulenyl]-alt-[1ꞌꞌ,3ꞌꞌ-azulenyl] 5. The photophysical and thermal properties of the obtained fluorene-azulene polymers were characterized by UF-Vis and fluorescence spectroscopy, as well as methods of thermogravimetric analysis and differential scanning calorimetry. It was found that polymers 3 and 5 do not emit in a neutral state in various organic solvents. However, they become fluorescent on treatment with trifluoroacetic acid in tetrahydrofuran. The “activation” of fluorescence in protonated azulene copolymers 3-H⁺ and 5-H⁺ occurs due to the formation of the 6π-electron aromatic azulene cation, which changes the overall electronic character, in particular, the levels of the frontal molecular orbitals of the HOMO and LUMO, and then the band gap. The obtained results demonstrate that the inclusion of azulene structural units in the conjugated polymer strucrure has great potential for the development of new protonic organic functional materials.