Twists and Turns Around a Square: The Many Faces of Squaraine Chemistry

Abstract

The purpose of this account is to give an overview of the wide variety of available structures that share the common name 'squaraines', with particular attention to the results achieved by our group in the last ten years. The first section of the report is an introduction and a description of the motivation for our interest in squaraine compounds. The second section includes a survey of the history of squaraines in the scientific literature, and the third section contains a general description of their synthesis and reactivity, together with reports on our direct experiences of their use in materials science, with a focus on squaraines in third-generation photovoltaics, nonlinear optics, and photodynamic therapy. In all of the cases, we highlight the establishment of structure-property relationships directed at the design of materials with improved properties.

Squaraine‐Based Polymers: Toward Optimized Structures for Optoelectronic Devices

“…Functional dyes have been widely investigated as molecular semiconductors in (opto)electronic devices, owing to their intense absorption and high quantum yield fluorescence emission at low energies . Among these materials, squaraine dyes have been the subject of many studies because of their remarkable donor–acceptor–donor (D–A–D) type structure, enabling systems with very small optical band gaps, and ultimately, unique photochemical/photophysical properties . Applications include organic solar cells, nonlinear optics, photodetectors, light‐emitting/ambipolar field‐effect transistors (FETs), and fluorescent labels/sensors for a wide range of analytes .…”

Pyrrolyl Squaraines–Fifty Golden Years

“…A detailed description of all of the precise squaraine variants can be found in [82]. The first published review of squaraine chemistry was reported later that same year by Maahs and Hegenberg [71], which included a section on pyrrolyl squaraines.…”

“…Additionally increasing throughout this period of time was the synthesis of unsymmetrical pyrrole squaraine dyes [19,39,48,53,57,69,70]. Although a synthetic route to the pyrrole half-squaraine, semisquaraine, or emisquaraine (the necessary precursor to an unsymmetrical pyrrole squaraine dye), was first proposed in 1968 [3], more efficient (and safer) routes have been developed in recent times (refer to [82] for a good description and schematics). Mention should also be added on the increase in recent years in the synthesis of (what in [82] is referred to as) core substituted squaraines, although only one example of this type of squaraine is known in a pyrrole squaraine [53].…”

“…Research involving pyrrolyl squaraines has been disseminated in the academic press for the last 50 years and in many ways the history of pyrrolyl squaraines is representative of the history of all squaraine compounds in that although during certain periods the focus of scientific endeavour has most certainly been on other squaraine variants, two of the first three squaraine types reported were pyrrolyl squaraines and some of the latest squaraine research involves pyrrolyl squaraines. Over this period of time a number of reviews have been written on squaraine compounds as a whole, some broad and all encompassing, some personal OPEN ACCESS research accounts, including within them, where relevant, mention of pyrrolyl squaraines [71][72][73][74][75][76][77][78][79][80][81][82], but thus far pyrrolyl squaraines, as their own unique subset of squaraine chemistry, have not been exclusively presented in a review (although a major portion of pyrrolyl squaraine chemistry can be learned by reading [74,78,82]). This article will attempt to be, as best it can, as concise and definitive as possible in terms of representing the papers that have been published on pyrrolyl squaraines and it will also attempt to collate relevant information from those publications to highlight certain important aspects of pyrrolyl squaraine chemistry.…”

Synthesis and Characterization of Squaraine‐Based Photocrosslinkable Resists for Bulk Heterojunction Solar Cells

“…[32,[42][43][44][45][46][47][48][49][50][51][52][53][54][55] In particular, we considered one of the simplest squaraine structures -the anilino-squaraines -to demonstrate the method (thereby providing proof of concept data) rather than focusing at this juncture on attaining particularly high conversion efficiencies. The choice of squaraines (the 1,3-dicondensation products of squaric acid and two equivalents of electron rich precursors) among all possible donor materials described in the literature is due to the fact that such compounds offer a good trade off between synthetic accessibility and overall conversion efficiencies.…”