Synthesis and Spectroscopic Characterization of New NIR Absorbing (2‐Thienyl)‐ and (4‐Dialkylaminoaryl)‐substituted Croconic Acid Dyes

Abstract

By condensation of croconic acid (7) with 2‐(dialkylamino)‐thiophenes 6a–d and with 3‐(dialkylamino)phenols 9a–c the deeply colored, sparingly soluble croconic acid dyes (“croconines”) 8a–d and 10a–c, respectively, have been prepared. These croconines have been characterized by UV/Vis, IR, and 1H‐NMR spectrometry. They exhibit strong electronic absorption bands in the near infrared (NIR) region (λmax ≈ 750–850 nm; cf. Table 1).

Croconaine Dyes – the Lesser Known Siblings of Squaraines

“…Scheme 6. Following on the multiple reports in 1992, 1993 again saw an increase in the number of publications on croconaine dyes. Two papers published in 1993 included one continuing an investigation on polycroconaines 8 and 9, [26] and the other covering croconaine dyes of types 13 and 16 (λ max > 786 nm), [27] prepared by refluxing in butanol/toluene mixtures (Scheme 7). [22] The carbazolium croconaine 14 in the first patent from 3M [21] was prepared in a mixture of quinoline/butanol/benzene (-H 2 O), similar to the original syntheses of the squaraine analogues of 2-5.…”

“…[61][62][63][64][65][66] Of the three papers in 2007, one was a theoretical paper covering type 2, 4, 5, 13, 16, 23 and 24 croconaines, [58] while the other two were experimental papers, both covering type 4 [59,60] and 4b [59] croconaines, prepared by variants of previously reported methods. [7,27] Of the six papers in 2008, there were four experimental papers, one theoretical paper and one hybrid paper. The theoretical paper [61] covered types 2-5, 13, 16 and 23-26 and the hybrid paper [62] covered ten compounds of type 23 and 28 croconaines prepared in warm methanol, whereas of the four experimental papers, two covered type 5 croconaines prepared under reflux in ethanol/ quinoline mixtures [λ max > 840 nm (DMF)], [63,64] one covered a type 13 croconaine prepared by a previous method [7] [λ max = 823 nm (CH 2 Cl 2 )], [65] and the fourth focussed on a water-soluble type 16 croconaine prepared by a previously reported method [27] [λ max = 783 nm (MeOH)].…”

“…[7,27] Of the six papers in 2008, there were four experimental papers, one theoretical paper and one hybrid paper. The theoretical paper [61] covered types 2-5, 13, 16 and 23-26 and the hybrid paper [62] covered ten compounds of type 23 and 28 croconaines prepared in warm methanol, whereas of the four experimental papers, two covered type 5 croconaines prepared under reflux in ethanol/ quinoline mixtures [λ max > 840 nm (DMF)], [63,64] one covered a type 13 croconaine prepared by a previous method [7] [λ max = 823 nm (CH 2 Cl 2 )], [65] and the fourth focussed on a water-soluble type 16 croconaine prepared by a previously reported method [27] [λ max = 783 nm (MeOH)]. [66] Three further papers were published in 2009 and 2010, which were all theoretical studies on type 13, 23, 24, [67] 16 [68] and type 13 croconaines, [69] before the next patent appeared (Scheme 13).…”

“…Thirty croconaines of types 1, 2, 4, 5, 11, 13, 17, 18 and 29 were covered in a patent from 2011 by Fujifilm. [27] In 2012, the synthesis of both squaraine and croconaine analogues of 30 were reported to exhibit a λ max of 1058 nm for the squaraine dye and 1052 nm for the croconaine dye, which is probably the first time in squaraine/croconaine history that the identical squaraine analogue exhibited a λ max at higher wavelengths compared to the corresponding croconaine. [27] In 2012, the synthesis of both squaraine and croconaine analogues of 30 were reported to exhibit a λ max of 1058 nm for the squaraine dye and 1052 nm for the croconaine dye, which is probably the first time in squaraine/croconaine history that the identical squaraine analogue exhibited a λ max at higher wavelengths compared to the corresponding croconaine.…”

“…[27] In 2012, the synthesis of both squaraine and croconaine analogues of 30 were reported to exhibit a λ max of 1058 nm for the squaraine dye and 1052 nm for the croconaine dye, which is probably the first time in squaraine/croconaine history that the identical squaraine analogue exhibited a λ max at higher wavelengths compared to the corresponding croconaine. The first croconaine/tetralactam-macrocycle rotaxane 31 (λ max = 830 nm) was reported in 2013 [72] and involved a type 16 croconaine (λ max = 795 nm), prepared by a previous method [27] under an argon atmosphere. The first croconaine/tetralactam-macrocycle rotaxane 31 (λ max = 830 nm) was reported in 2013 [72] and involved a type 16 croconaine (λ max = 795 nm), prepared by a previous method [27] under an argon atmosphere.…”