Effect of Protonation on Optical and Electrochemical Properties of Thiophene–Phenylene-Based Schiff Bases with Alkoxy Side Groups

Abstract

Three polyazomethines and their corresponding model compounds were protonated with trifluoroacetic acid, and its effect on their optical (UV–vis absorption and photoluminescence) properties and electrochemical behavior has been studied, in the context of the presence and elongation of alkoxy side groups. Moreover, the effect of environment dielectric constants (i.e., polarity of the solvent) was considered on the doping process. It has been proven that the presence of alkoxy side groups is necessary for protonation to occur, while unsubstituted compounds undergo hydrolysis to constitutive units. Acid doping of imines consisting of alkoxy side chains has resulted in a distinct bathochromic shift (>200 nm) of the low-energy absorption band. Even the length of alkyl chains has not affected the position of shifted bands; it has been observed that azomethines with smaller, methoxy side groups undergo the protonation process much faster than their octyloxy-substituted analogues, due to the absence of steric hindrance. The electrochemical studies of these alkoxy-substituted imines have indicated a better p-type behavior after protonation induced by the capability of the protonated form to easily oxidize in acetonitrile and to generate the native molecules. The environmental polarity has also had impact on the doping process, which took place only in low-polar media.

A primer to in-situ opto- and spectro-electrochemistry

“…UV-Vis-NIR spectroscopy can be applied to detect changes in the electronic structure (i.e. color changes) of a molecule during redox (Figure 4c,e) [32,33], protonation/deprotonation [34], ligand coordination/dissociation (in organometallic complexes, Figure 4d,g) [35], or polymerization [36]. Either transmission or reflection can be collected in the range of 135e3300 nm, and used for quantitative analysis.…”

N‐Doped Octagon‐Containing HBC as Redox and pH Chiroptical Switch in the NIR

“…[12] In particular, the π-conjugation on the graphenic structures can be modulated by nitrogen doping due to the lone pair of electrons provided. [13] It also allows the coordination with metals [14] or protons, [15,16] which will lead to ligands in supramolecular structures or acidresponsive materials, respectively. For instance, it has been shown that the incorporation of pyrimidine [17] or pyridine [18] rings into the HBC skeleton not only modifies the electronic properties but also allows the modulation of their properties with pH variations.…”

Direct Arylation Polymerization of Degradable Imine-Based Conjugated Polymers

“…52 It has been previously demonstrated that imine bonds can undergo protonic acid-doping, in which the imine bond is protonated, favoring the formation of a proposed quinoidal structure of the polymer with exocyclic double bonds (Figure 3a). 53,54 Polymers that favor the quinoidal structure have been investigated for their application in n-type thermoelectric devices and n-type OFETs as they exhibit a profound narrowing of the HOMO−LUMO bandgap and exhibit lower-lying LUMO compared to their benzenoidal counterparts. 55−58 Although 2% v/v TFA showed a prominent shift, the presence of this lower energy absorbance peak begins to emerge at 0.03% v/v TFA.…”

“…Overall, these observations were consistent with the protonation of other imine-based conjugated polymers documented in the literature. 22,52,53,60 The progression of degradation over time was investigated spectroscopically for samples treated with low (0.03%) and high (0.35%) v/v TFA (Figure 3d,e). Although a minor shoulder of ca.…”

“…We hypothesize that the bathochromic shift, forming a lower energy peak at 778 nm, corresponds to the protonated quinoidal structure of the polymer due to the increased electron deficiency when the nitrogen of the imine is protonated . It has been previously demonstrated that imine bonds can undergo protonic acid-doping, in which the imine bond is protonated, favoring the formation of a proposed quinoidal structure of the polymer with exocyclic double bonds (Figure a). , Polymers that favor the quinoidal structure have been investigated for their application in n-type thermoelectric devices and n-type OFETs as they exhibit a profound narrowing of the HOMO–LUMO bandgap and exhibit lower-lying LUMO compared to their benzenoidal counterparts. − Although 2% v/v TFA showed a prominent shift, the presence of this lower energy absorbance peak begins to emerge at 0.03% v/v TFA. At 0.09% v/v TFA, the original p­(IDT–TIT) peak at 570 nm was accompanied by approximately an equal intensity peak at 725 nm, indicating partial acid-doping, where there is an equilibrium between the undoped benzenoidal structure and the doped quinoidal structure.…”