Synthesis, electronic, and emission spectroscopy, and electrochromic characterization of azulene-fluorene conjugated oligomers and polymers

Two model compounds, l,3-bis(9,9-dihexylfluoren-2-yl)azulene (M1), and l,3-bis[7-(9,9,9′,9′-tetrahexyl-2,2′-bifluoren-7-yl)azulene (M2), and polymers, poly[2,7-(9,9-dialkylnuorenyl)-alt-(l′, 3′azulenyl)](P1-P4)andpoly{[1,3-bis(9′,9′-dihexylfluoren- 2′-y1)azulenyl]-alt-[2′,7″9″,9″-dialkylnuorenyl]} (P5, P6) were synthesized by reacting 1,3-dibromoazulene or l,3-bis(7-bromo-9,9- dihexylfluoren-2-yl) azulene with a suitable 9,9-dialkylflourenyl-2-borate or 2,7-diborate via Suzuki cross-coupling reactions. The thermal and optical properties of the polymers were characterized by thermogravimetric analysis, differential scanning calorimetry, and UV-vis and fluorescence spectroscopy. M1, M2, and P1-P6 are nonfluorescent in the neutral state in different organic solvents. However, M1, M2, P5, and P6 become fluorescent upon the addition of trifluoroacetic acid in THF, with relative quantum efficiencies of 0.0040.06. The "switching on" of the fluorescence for the azulene-fluorene copolymers and model compounds is due to the formation of a 6π electron aromatic azulenium cation, which alters the overall electronic character, particularly, the HOMO and LUMO, and subsequently the band gap. Cyclic voltammograms of polymer films prepared by spin-coating polymer solution onto an indium-tin oxide-coated glass substrate showed that all of the polymers films exhibited the relatively low half-wave oxidation potentials in the range 0.840.93 V vs Ag/AgCl. Electrochromic devices of P3 and P5 with a sandwich structure of PET-ITO/polymer/ PMMA-PC-LiC1O₄/PET-ITO were fabricated, and color changes from light yellow to brown and from yellow to green for polymers P3 and P5 films, respectively, are observed.