Croconaine Dyes – the Lesser Known Siblings of Squaraines

Abstract

In the 50+ years since the first squaraine dye, made by using 3,4‐dihydroxycyclobut‐3‐ene‐1,2‐dione (squaric acid), was reported, numerous analogues have not only been made and published, but similarly, a number of good review articles on squaraine dyes has also been published. In contrast, for the majority of this time, reports of croconaine dyes, made by using 4,5‐dihydroxy‐4‐cyclopentene‐1,2,3‐trione (croconic acid), have also been published, but they have not received the same attention as squaraine dyes. Furthermore, the only known review of croconaine dyes, which was recently published, is written in Chinese. This current review aims to document the history of croconaine dyes since their first report in a German patent in 1970. This review sources both the scientific literature and published patents, and attention is given to both synthesis and spectroscopic details.

Croconaine Rotaxane Dye with 984 nm Absorption: Wavelength‐Selective Photothermal Heating

“…While some croconaine dyes are known with absorption maxima between 900–1000 nm, none have been reported as an interlocked rotaxane . The chemical structure of croconaine C1 has two flanking thiophene units and in a recent study of analogous squaraine dyes, we found that synthetic substitution of the thiophenes with flanking thienothiophene units led to an increase in conjugation and a 100 nm red‐shift in the squaraine absorption maxima band .…”

“…While some croconaine dyes are known with absorption maxima between 900-1000 nm, none have been reported as an interlocked rotaxane. [15] The chemical structure of croconaine C1 has two flanking thiophene units and in a recent study of analogous squaraine dyes, we found that synthetic substitution of the thiophenes with flanking thienothiophene units led to an increase in conjugation and a 100 nm red-shift in the squaraine absorption maxima band. [27] Equally important, the flanking thienothiophene units still allowed threading of the tetralactam macrocycle M. With this precedence in mind we decided to prepare the new extended croconaine dye C2 and determine if it could be used to thread M and be converted into the new croconaine rotaxane 2.…”

“…gaining increasing attention for various sensing, photothermal, optoelectronic, and photoacoustic applications. [15][16][17][18] In 2013, we showed that croconaine dyes, such as C1 (Scheme 1), are very useful as near-infrared photothermal agents, [14] and in the subsequent time period we and others have shown that they can be used for clean photothermal heating of nanoparticles without producing singlet oxygen, [19] photothermal switching of polymer morphology, [20,21] and laser ablation of tumors in living animals. [22,23] We have also shown that croconaines can be converted to rotaxane structures, such as 1, that exhibit sharp absorption bands at 820 nm, even under conditions that aggregate the molecules.…”

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

“…In the 50 plus years since the synthesis of the first generations of squaraine dyes, there is 1 year that stands out as a marker of significantly increased interest, especially in the patent literature whose purpose is to cover potential commercial applications. The significance of the year 2000 was recently discussed in microreviews concenring both azulenyl squaraine dyes and croconaine dyes : the turn of the millenium saw the first mass production of blue laser diodes. Previously, all optoelectronic applications had been designed for red laser diodes and dyes that absorbed in the visible–near infrared region of the spectrum (ideally 700–900 nm) and, accordingly, dyes that absorbed in this region of the spectrum (including squaraine dyes) received much attention from the early 1970s to around the year 2000.…”

Potential Use of Squarates and Croconates as Singlet Fission Sensitizers

“…The particular electronic structure of these dyes confers interesting features appealing for technological applications, like flexibility, sharp and intense absorption of light as well as photostability. [4,5,6] Hence, optoelectronics is one of the major applications for which the investigations on these compounds have been directed. For example, squarate dyes have motivated a considerable amount of investigations in fields like data storage, [7] fluorescence labeling [8,9,10,11] and nonlinear optics.…”