Origin of Near-Infrared Absorption for Azulene-Containing Conjugated Polymers upon Protonation or Oxidation Tao Tang, † Tingting Lin, §̧ FuKe Wang,* ,‡,§̧ and Chaobin He* ,†,§̧ † Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore, Singapore 117576 ‡ Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore, Singapore 117543 §̧ Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore, Singapore, 117602 * S Supporting Information ABSTRACT: A series of azulene-containing conjugated polymers were studied to elucidate their tunable absorption properties in near-infrared (NIR) regions (i.e., 1.2-2.5 μm) upon protonation/oxidation. Density function theory (DFT) revealed that protonation-induced intramolecular charge transfer (ICT) in the polymer backbone lead to strong NIR absorption. Distinct spectral change was observed when tiny amount of peroxide was added to the protonated polymer in trifluoroacetic acid/chloroform solution. Electron paramagnetic resonance (EPR) study confirmed the presence of radical cation, which results in the occurrence of newly formed absorption band after the addition of peroxide. The spectro-electrochemical results and DFT study indicate that polarons and polaron pairs were formed during p-doping process, and both the chemical oxidation and electrochemical oxidation could be facilitated by TFA protonation. This represents the first reported mechanisms of NIR absorption under various protonation/oxidation conditions in a single polymer system. ■ INTRODUCTION Azulene and its derivatives have received a plethora of interests during the past a few decades due to its unique electronic and structural characters such as a dipolar structure with the dipole moment around 1.0 D, high electron affinity, low ionization energy and unusual S 2 -S 0 fluorescence. 1-8 The large dipole moment arises from the electron drift from the seven- membered ring to five-membered ring, which could be considered as an aromatic 6π electron tropylium cation fused by a 6π electron cyclopentadienyl anion. 9,10 This remarkable electronic structure of azulene allows for cation stabilization through aromatization of the seven-membered ring, which has promising applications such as electrochromic or switching materials, 11 NIR sensor 12 and conducting polymers. 13 Our involvement of azulene and its derivatives stems from our interest in preparing solution-processable polymers with tunable NIR absorption via a simple way, which is widely used in the areas of telecommunication, defense, medical therapy, heat absorbers, and optical recording. 14-17 Common strategy to achieve low band gap conjugated chromophores is to adopt donor-acceptor (D-A) configuration, and this has become a widely used method for synthesizing conjugated polymers with narrow band gap through tailoring the donor and acceptor units. 18-21 Furthermore, methods such as protonation, 10,22 chemical oxidation, 23 Lewis acid adducts, 24,25 and electro- chemistry 26 are also used to achieve desired low band gap chromophores. It has also been demonstrated that anodic potentials or strong oxidants can lead to the decrease of band gap and occurrence of NIR absorption. 13,27 A commonly accepted model for this phenomenon is the formation of polarons, bipolarons and polaron pairs with a nondegenerate ground state. 28,29 A polaron is simply a radical cation or anion which is associated with a geometrical deformation delocalized over a limited number of monomeric units. 26 When doping level is increased, an extra polaron is formed. These two polarons can coalesce into a bipolaron. However, polaron pairs are generated if the distance between the two polarons is large enough to inhibit the formation of bipolaron. Polarons are also the essential charge carriers in sensors, solar cells and electro- chromic devices (ECD) with great electronic effect. 30 Conjugated polymers such as polythiophenes and polyfluorenes with eletrochromic behavior are extensively studied due to their good conductivity, stability, and high contrast. Under anodic or cathodic potential, these polymers all exhibit reduced bandgap with extending the maximum absorption into NIR range. Recently, we have investigated a series of azulene containing chromophores and polymers with tunable NIR absorption ranging from 1.2 to 2.5 μm after protonation, 22,31,32 which also exhibit outstanding spectroscopic properties and thermal stability. Unlike the occurrence of NIR absorption in Received: February 17, 2015 Revised: May 20, 2015 Published: May 20, 2015 Article pubs.acs.org/JPCB © 2015 American Chemical Society 8176 DOI: 10.1021/acs.jpcb.5b01613 J. Phys. Chem. B 2015, 119, 8176-8183