Tuning the Photophysical Properties of Acceptor–Donor–Acceptor Di-2-(2-oxindolin-3-ylidene) Malononitrile Materials via Extended π–Conjugation: A Joint Experimental and Theoretical Study

Abstract

Many optoelectronic applications require organic semiconductor (OSC) materials with high electron affinity. In this work, a series of novel acceptor–donor–acceptor (A–D–A) materials with low-lying LUMO energy levels were designed and characterized. In this strategy, two acceptor dyes, bis-isatin and di-2-(2-oxindolin-3-ylidene) malononitrile, were connected by various π–bridges (benzene ring, benzo[c][1,2,5]thiadiazole, monothiophene, trithiophene). We varied the length of the π–conjugation of the central core and the linkage position of the acceptor core (4- vs. 6-position of the phenyl ring) to investigate the effect on the optical and electrochemical properties of the materials. We performed density functional theory (DFT) and time-dependent DFT (TD–DFT) studies to gain insight into the dyes’ electronic properties by determining the energy levels. Our findings demonstrate that with increasing acceptor strength and π–conjugation length of the core, the wavelength of the longest absorption maximum as well as their respective extinction coefficients are enhanced, which results in band-gap reduction either by lowering the LUMO and/or raising the HOMO energy level of the molecules. The potential practical utility of these materials as electron-transport materials for perovskite solar cells (PSCs) has been demonstrated.

Design of Novel Functional Conductive Structures and Preparation of High-Hole-Mobility Polymer Transistors by Green Synthesis Using Acceptor–Donor–Acceptor Strategies

“…Typical studies of macromolecular semiconductor materials have been carried out in relation to their good chemical modifiability and functionalization capabilities, as well as to their good film-forming properties, which distinguish them from small molecules [11][12][13]. Compared to the donor-donor type or acceptor-acceptor type polymers, the donor-acceptor type materials possess better modulation of the frontier molecular orbital (FMO) levels and thus are promising potential materials for device-oriented applications [14][15][16]. The FMO energy levels include the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels, which are dominated by the type and content of donor and acceptor in the molecular architecture, respectively [17,18].…”