Isomerization and Properties of Isomers of Carbocyanine Dyes
|Reviewer 1 Alexander Ishchenko National Academy of Sciences of Ukraine, Kiev, Ukraine||Reviewer 2 Radosław Podsiadły Institute of Polymer and Dye Tehnology, Lodz University of Technology|
Approved with revisions
|Version 2||Reviewer invited|
Published: 29 November 2018
AbstractOne of the important features of polymethine (cyanine) dyes is isomerization around one of C–C bonds of the polymethine chain. In this review, spectral properties of the isomers, photoisomerization and thermal back isomerization of carbocyanine dyes, mostly meso-substituted carbocyanine dyes, are considered. meso-Alkyl-substituted thiacarbocyanine dyes are present in polar solvents mainly as cis isomers and, hence, exhibit no photoisomerization, whereas in nonpolar solvents, in which the dyes are in the trans form, photoisomerization takes place. In contrast, the meso-substituted dyes 3,3′-dimethyl-9-phenylthiacarbocyanine and 3,3′-diethyl-9-(2-hydroxy-4-methoxyphenyl)thiacarbocyanine occur as trans isomers and exhibit photoisomerization in both polar and nonpolar solvents. The behavior of these dyes may be explained by the fact that the phenyl ring of the substituent in their molecules can be twisted at some angle, removing the substituent from the plane of the molecule and reducing its steric effect on the conformation of the trans isomer. In some cases, photoisomerization of cis isomers of meso-substituted carbocyanine dyes is also observed (for some meso-alkyl-substituted dyes complexed with DNA and chondroitin-4-sulfate; for 3,3′-diethyl-9-methoxythiacarbocyanine in moderate polarity solvents). The cycle photoisomerization–thermal back isomerization of cyanine dyes can be used in various systems of information storage and deserves further investigation using modern research methods.
Share & Cite This Article
Pronkin, P.; Tatikolov, A. Isomerization and Properties of Isomers of Carbocyanine Dyes. Sci 2019, 1, 5.
Pronkin P, Tatikolov A. Isomerization and Properties of Isomers of Carbocyanine Dyes. Sci. 2019; 1(1):5.Chicago/Turabian Style
Pronkin, Pavel; Tatikolov, Alexander. 2019. "Isomerization and Properties of Isomers of Carbocyanine Dyes." Sci 1, no. 1: 5.
Note that from the first issue of 2016, MDPI journals use article numbers instead of page numbers. See further details here.
National Academy of Sciences of Ukraine, Kiev, Ukraine
This review presents us a thorough analysis of the literature data on (photo)isomerization processes in a theoretically and practically important subclass of polymethine dyes — cationic carbocyanines. All the factors influencing their isomerization is logically classified and discussed: bulky substituents in the polymethine chain, solvent viscosity, solvent polarity, formation of ion pairs and dye aggregates, interaction with highly organized structures, e.g. DNA, etc.
I believe that, regardless of the fact that a good deal of the cited literature was published some 20 and more years ago, this is relevant and useful publication, which can help one to systematize the knowledge on isomerization of polymethine dyes. And presently new researches in this field is imminent, utilizing the improved time-resolved physical-chemical technics and modern quantum chemical methods, e.g. QM/MM explicit solvation models.
There are still some minor remarks to the paper.
• The authors use non-IUPAC, somewhat obsolete, nomenclature of the carbocyanine dyes, which could be confusing for a general reader. Moreover, they do not give the structural formulas of the discussed dyes in the paper, only their names, which hampers the perception of the material. Therefore, I suggest modifying Figure 1, to show in it the actual structures of dyes to be discussed below instead of the examples of terminal groups.
• Subsection 2 of the paper (Structure and Spectral-Fluorescent Properties of Cyanine Dyes) should be slightly modified. Namely, its last paragraph should be moved to its very beginning, so that a reader had not been confused why only cationic polymethines are discussed. It should be noted also, that even the subclass of cationic polymethine dyes is not limited to the structures with "terminal nitrogen atoms". There are, for example, pyrilocyanines (also (thia,selena)pyrilocyanines) and even carbocyclic azulenecyanines.
• The authors should think about improving the nomenclature of isomers (see Figure 3, for example). The point is that the EZ-names differ for the same isomers of carbocyanines with different terminal groups. Thus, all-trans isomers of inda- dyes are EEEE, but those of thia- and oxa- dyes are ZEEZ. Not mentioning the fact that with meso-methoxy or meso-chloro substituent in the polymethine chain, the all-trans isomer of thiacarbocyanine will be ZZZZ.
• There is an interesting fact worth of discussing, namely the vast difference of the reverse thermal isomerization constants for rather similar dyes 2 and 3.
• Page 6: there is a misprint in the eq.3. Thus, one can read at page 7 that according to this equation "the reaction rate constant should be inversely proportional to the viscosity of the solvent". However, viscosity is not present in this equation (while it is in the corresponding equation in Ref.36).
• Russian-language book in Ref.17 is inaccessible to most of the readers. Therefore, I would suggest replacing it with the reference to the relative review: Ishchenko, A.A. Structure and spectral-luminescent properties of polymethine dyes. Russ. Chem. Rev. 1991, 60, 865–884, doi:10.1070/RC1991v060n08ABEH001116.
Response to Reviewer 1Sent on 26 Mar 2019 by Pavel Pronkin, Alexander Tatikolov
Institute of Polymer and Dye Tehnology, Lodz University of Technology
In my opinion authors have presented very well the topics which are focused on photoisomerization of cyanine dyes.