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

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.