Design, synthesis and functionalization of spiro-annulated and boron-containing [pi]-conjugated donor-acceptor molecules and their application studies in organic electronics

Abstract

A series of donor–π–donor (D–π–D) or donor–acceptor–donor (D–A–D) small molecules based on a judiciously designed spirothienoquinoline moiety as the donor moiety has been synthesized and characterized. This series of compounds were found to exhibit high-lying highest occupied molecular orbital (HOMO) energy levels. The X-ray crystal structure analyses as well as the computational studies revealed an enhanced quinoidal character along the D–A–D molecular backbone. These, together with their excellent thermal and electrochemical stabilities, have allowed these spirothienoquinoline-based small molecules to serve as electron donor materials in vacuum-deposited bulk heterojunction (BHJ) organic solar cells (OSCs) with a low donor dopant concentration and as active materials in organic resistive memory devices. Respectable photovoltaic performances with power conversion efficiencies (PCEs) of up to 3.86% and binary logic memory performances have been demonstrated, respectively. These results indicated that the spirothienoquinoline moiety could be a potential building block for constructing multifunctional molecular materials. Extension of the work to the design and synthesis of a series of dendritic spiroacridine-based compounds with different hole-transporting arylamine moieties has been made. Their thermal stabilities, photophysical, electrochemical as well as photovoltaic properties have been explored, which were believed to provide insights into the effects of hole-transporting properties of the electron donor materials on the performance of BHJ OSCs with a low donor dopant concentration.

To develop high-performance boron-containing non-fullerene acceptors (NFAs) for solution-processed BHJ OSCs, a series of acceptor-donor-acceptor (A–D–A) small molecules bearing the boron β-diketonate-based acceptor moiety has been synthesized and characterized. Chemical modifications on the central donor moiety and the terminal boron β-diketonate-based acceptor moiety have shown to greatly affect their photophysical and electrochemical properties. By blending this series of compounds with polymeric electron donors, good photovoltaic responses with PCEs of up to 4.31% have been achieved. Moreover, the applicability of some of the compounds as active materials in organic resistive memory devices has also be examined, in which binary logic memory performances have been realized. To gain more insights into the underlying working mechanism of the organic resistive memory devices fabricated with boron β-diketonate-containing materials, another series of donor–π–acceptor (D–π–A) small molecules has been synthesized and characterized, and the mechanism of their resistive switching behavior has been studied.

A class of four-coordinate fluoroboron emitters with a rigid tridentate 2,2'-(pyridine-2,6-diyl)diphenolate (dppy) ligand featuring thermally activated delayed fluorescence (TADF) properties and tunable emission colors has been synthesized and characterized. Their photophysical and electrochemical properties have been studied and the involvement of TADF processes in the excited states have been supported by computational studies. High external quantum efficiencies (EQEs) of up to 18.0% and long operational stabilities of the green-emitting organic light-emitting diodes (OLEDs) with half-lifetimes of up to 12733 hours at an initial luminance of 100 cd m–2 have been achieved. These findings demonstrated for the first time the design and synthesis of stable and efficient TADF emitters based on four-coordinate boron-containing compounds with a tridentate dppy ligand.