Synthesis, Electronic, and Emission Spectroscopy, and Electrochromic Characterization of Azulene−Fluorene Conjugated Oligomers and Polymers

Abstract

Two model compounds, 1,3-bis(9,9-dihexylfluoren-2-yl)azulene (M1), and 1,3-bis[7-(9,9,9′,9′-tetrahexyl-2,2′-bifluoren-7-yl)azulene (M2), and polymers, poly[2,7-(9,9-dialkylfluorenyl)-alt-(1′,3′-azulenyl)] (P1−P4) and poly{[1,3-bis(9′,9′-dihexylfluoren-2′-yl)azulenyl]-alt-[2′′,7′′-(9′′,9′′-dialkylfluorenyl]} (P5, P6) were synthesized by reacting 1,3-dibromoazulene or 1,3-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 0.004−0.06. 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 0.84−0.93 V vs Ag/AgCl. Electrochromic devices of P3 and P5 with a sandwich structure of PET-ITO/polymer/PMMA-PC-LiClO4/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.

Modulating the Properties of Azulene‐Containing Polymers through Controlled Incorporation of Regioisomers

“…[ 28,35,44 ] In addition, each of the model compounds exhibit broad absorptions centered at approximately 650 nm characteristic of the S 0 -S 1 transition of azulene. [ 41 ] To gain additional insight into the effects of substitution and regiochemistry on the electronic structure and photophysical properties of the model compounds, density functional calculations were performed at the B3LYP/6-31G* level of theory (calculations for MT3 and MT4 are shown in Figure 3 ; for MF1 and MF2 , see Figure S1 in the Supporting Information). In the presence of strong acids, protonation of the 5-membered ring of azulene leads to the formation of an aromatic, 6π-electron tropylium cation.…”

“…In comparison to previous work on random conjugated polymers, we envisaged that materials with varied optoelectronic properties could be accessed through copolymerization of multiple monomer units; however, instead of using signifi cantly disparate building blocks, the electronic and structural properties of the polymers could be tuned simply by varying the regiochemistry of the azulene units along the polymer chain. [ 41 ] A second polymer series ( PT1 -PT5 ) was also prepared, again reacting varying feed ratios of dibromoazulene regioisomers 1 and 2 with bis-stannylated bithiophene monomer 4 under Stille cross-coupling conditions using Pd 2 dba 3 and P( o -tol) 3 in toluene at 120 °C. For the fi rst series ( PF1 -PF5 ), the commercially available 9,9-dioctylfl uorene-2,7-diboronate monomer 3 was copolymerized with varying feed ratios of 1 and 2 under Suzuki-Miyaura cross-coupling conditions using Pd 2 dba 3 and P( o -tol) 3 in toluene at 85 °C with tetraethylammonium hydroxide as a boron-activating base.…”

“…[30][31][32] In the case of azulene-containing materials, protonation leads to the formation of a stable tropylium cation with signifi cantly different spectroscopic properties than the neutral species. [37][38][39][40][41][42][43] We recently demonstrated that azulene-based materials connected through the electron-poor 7-membered ring (4,7-connectivity) exhibit unique properties, in part due to the conservation of π-conjugation after protonation. [37][38][39][40][41][42][43] We recently demonstrated that azulene-based materials connected through the electron-poor 7-membered ring (4,7-connectivity) exhibit unique properties, in part due to the conservation of π-conjugation after protonation.…”

Origin of Near-Infrared Absorption for Azulene-Containing Conjugated Polymers upon Protonation or Oxidation

“…10,35,36 Therefore, upon the treatment of trifluoroacetic acid (TFA), π-electron drift from the sevenmembered ring of azulene to the five-membered ring generates cycloheptatrienyl (tropylium) cation and makes azulene become a strong electron-acceptor. 2,37 Significant redistribution of charge is formed and generates distinct charge separation. As a result, substantial charge transfer from HOMO to LUMO after excitation leads to strong ICT which could lower the bandgap and result in long wavelength absorption.…”

Stimuli‐Responsive Cyclopenta[ef]heptalenes: Synthesis and Optical Properties

“…[19] It can be seen that the energy gap decreases through the series of derivatives. These values are consistent with the difference between the HOMO and LUMO energies determined through www.eurjoc.orgquantum chemical calculations, and similar to other azulene derivatives previously reported.…”