Structure–properties relationship in diketopyrrolopyrrole based small molecules using functional terminal side chains via direct arylation: a joint experimental and theoretical study

Abstract

In this article, we report a series of diketopyrrolopyrrole based donor–acceptor small molecules, PDPP-OC8, PDPP-EC8, PDPP-EG and PDPP-CNEC8, with different phenyl containing flexible terminal side chains such as alkoxy (–OC₈H₁₇), ester ((–CO)OC₈H₁₇), ester with glycol ((–CO)(O(C₂H₄O)₃–CH₃)) and cyano acrylate with an octyl side chain ((–CN)(CO)OC₈H₁₇), respectively, via direct arylation. A comparative analysis has been done on their opto-electronic properties such as absorption, HOMO, LUMO, band gap, thermal properties, packing nature and hole mobility. The physical properties of the four small molecules were studied using absorption spectroscopy, thermal gravimetric analysis, cyclic voltammetry, differential scanning calorimetry and charge carrier mobility measurements. In comparison to molecules with alkoxy terminal side chains, those with ester and cyano acrylate terminal side chains exhibit (i) a lowered optical band gap from 1.65 eV (alkoxy) to 1.43 eV (ester) and (ii) a broad absorption covering the solar spectrum from 500 to 800 nm. From the results we are able to conclude that the terminal side chains significantly alter their optical and electronic properties. Density functional theory calculations have been carried out to substantiate the opto-electronic characteristics and further understand the effects of terminal alkyl chains.