Pyrrolyl Squaraines–Fifty Golden Years

Abstract

Pyrrolyl squaraines, both dyes and polymers, were first reported in 1965 and since then a fascinating body of work has been produced investigating the chemistry of these interesting molecules. A major aspect of these molecules that makes them so appealing to those researchers who have contributed to this field over the last 50 years is their chemical versatility. In this review, subjects, such as the synthetic history, an understanding of the molecular structure, an overview of the optical properties, a discussion of both the electrical conduction properties, and magnetic properties, plus use of the particles of pyrrolyl squaraines, are presented. Furthermore, previously published results are not just presented; they are in certain cases collated and used to both highlight and explain important aspects of pyrrolyl squaraine chemistry.

Electro‐optical Properties of Neutral and Radical Ion Thienosquaraines

“…Condensation of N , N ‐disubstituted 2‐aminothiophenes 1 a – d with squaric acid leads to the formation of the desired thienosquaraines 2 a – d . The squaraine condensation reaction is known to be acid‐catalyzed by the squaric acid itself, which in most cases is strong enough to self‐catalyze its own reaction . This is an exceptional feature of SQ dyes compared with other class of optoelectronic materials, especially organic semiconductor materials, which are often prepared by metal‐catalyzed reactions.…”

Squaryl Molecular Metaphors – Application to Rational Drug Design and Imaging Agents

“…Since that time these pyrrolyl squaraines have generated interesting compounds, having both optical and electrical conduction properties in addition to magnetic properties [116]. The synthesis of pyrrolyl squaraines occurs between the condensation of a pyrrole (or linked bispyrrole) and squaric acid ( Figure 27).…”

Microreview: Pyrrol‐3‐yl squaraines (including indol‐3‐yl squaraines)

“…Pyrrolyl squaraines can further be subcategorized into 2‐yl and 3‐yl compounds (illustrated in Scheme ) with the definition arising from the positional contact from the pyrrole to the squarate core. The majority of pyrrolyl squaraines, including polysquaraines, thus far reported in both the scientific and patent literatures are pyrrol‐2‐yl‐derived symmetrical 1,3‐squarates . However, this microreview concentrates solely on the interesting facets of pyrrol‐3‐yl squaraine dye chemistry.…”

“…Squaraine dyes, compounds prepared via step‐wise condensation reactions of specific electron‐donating aromatic/heterocyclic ring systems and 3,4‐dihydroxycyclobut‐3‐ene‐1,2‐dione (squaric acid), have been extensively examined in both scientific and patent literatures since their first reported preparations in the late 1960s, with a number of review articles summarizing their many chemical/physical properties and applications . The vast majority of symmetrical squaraine dyes (where step‐wise condensation occurs in the same reaction pot) are represented by the six structural formulas in Scheme .…”