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

Paweł Nitschke,Mariana-Dana Damaceanu,Andra-Elena Bejan,Bożena Jarząbek

doi:10.1021/acs.jpcb.1c05390

Paweł Nitschke, Mariana-Dana Damaceanu + Show 2 more

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https://doi.org/10.1021/acs.jpcb.1c05390

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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.

Highlights

Conjugated compounds have drawn much attention in the last decades, owing to their interesting properties, well suited for application as organic semiconductors in many optoelectronic systems, such as organic photovoltaics,[1,2] organic light-emitting diodes,[3,4] or field effect transistors.[5]One group of these materials is represented by azomethines and polyazomethines (PAzs), consisting of an imine bond (−CH N), which has proved to have an isoelectronic character with a vinylene bond.[6]
This study presents the results of spectroscopic and electrochemical investigations on tuning the optoelectronic properties of some PAzs and their model compounds upon protonation with trifluoroacetic acid (TFA)
The effect of chemical structure variation, which is the substitution with alkoxy side groups and their length, on the doping process has been considered

Summary

Introduction

One group of these materials is represented by azomethines and polyazomethines (PAzs) ( known as imines or Schiff bases), consisting of an imine bond (−CH N), which has proved to have an isoelectronic character with a vinylene bond.[6] Such materials are much easier to synthesize, in contrary to carbon−carbon or vinylene-coupled compounds, which usually require stringent reaction conditions and rather extensive purification processes.[7] Azomethines (PAzs) are products of condensation (polycondensation) reaction between amines and aldehydes Such reaction may proceed under mild conditions, using inorganic or organic catalysts[8] or even without using any, as previously shown.[9,10] These types of materials were extensively studied due to their interesting thermal,[11] electrical,[12] and optical[13−15] properties and due to the possible activity in some optoelectronic systems, such as bulk heterojunction photovoltaic cells,[16−19] perovskite solar cells,[20−22] organic light-emitting devices,[23,24] or electrochemical systems.[25,26]

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