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. 2024 Aug 13;7:180. doi: 10.1038/s42004-024-01256-6

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© The Author(s) 2024

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 3 — Cyanine dyes are delineated as cationic compounds characterized by two terminal nitrogen aromatic units interconnected by a polymethine chain. The classification of cyanine dyes is contingent upon the length of the polymethine chain, leading to distinctions such as apocyanine, monomethine, trimethinine, pentamethine, and heptamethine cyanines. Those cyanine dyes incorporating a squaraine group within the polymethine chain are denoted as squarylium cyanines. Notably, the structural configuration of unsymmetric hemicyanines (e.g., dimethine and tetramethine) may encompass solely one nitrogen aromatic unit, setting them apart from their counterparts.