Substituent Control of Near-Infrared Absorption of Triphenylamine Radical Cation

Round 1

Reviewer 1 Report

I revised the Manuscript ID: colorants-1812285 entitled “Substituent Control of Near-Infrared Absorption of Triphenyl- 2 amine Radical Cation” submitted to Colorants by Yano et. al.

In this work, the authors investigated the possibility of five triphenyltriphenylamines, with various substituents, as precursors for near-infrared absorbing materials. None of the compounds is new, and their synthesis and characterization have already been described in the literature. I read this manuscript very carefully and I think the authors presents an important study of theorical calculations.

The authors present the synthesis of four compounds which is different from the syntheses already described in the literature. So, in my opinion, I do not see the need to have performed the HRMS spectra.

I left some notes throughout the manuscript that the author should pay attention to, in order to improve this review paper.

1- In the abstract, authors must indicate what the abbreviation DFT (Density functional theory) means as they present it for the CV.

2- On line 75, authors must indicate what the abbreviation EL means.

3- Authors should check the punctuation throughout the manuscript, since after many references the full stop is not found. Also, put the fullstop on line 98 (..nm. In...)

4- line 97 ...conclusions. )...change to ...conclusions)

5- line 127...Cyclic...change to ...cyclic...

6-line 133...Compopund.. change to ..compound..

7-line 160..1-5, Compound..1-5, compounds..

8-In figures 3 and 5 the authors have to put the red line for compound 2.

9-line 223 put the ...2... in bold

Given the above, I am of opinion that this paper should be accepted after the suggested corrections.

Author Response

Comments from Reviewer 1

The authors present the synthesis of four compounds which is different from the syntheses already described in the literature. So, in my opinion, I do not see the need to have performed the HRMS spectra.
Thank you for pointing this out. These are all known compounds, but are similar in structure to each other and show similar 1H and 13C NMR spectra. We also thought it necessary to prove that we were definitely obtaining the target product with C3 symmetry. For these reasons, HRMS were conducted.
 
1- In the abstract, authors must indicate what the abbreviation DFT (Density functional theory) means as they present it for the CV.
2- On line 75, authors must indicate what the abbreviation EL means.
3- Authors should check the punctuation throughout the manuscript, since after many references the full stop is not found. Also, put the fullstop on line 98 (..nm. In...)
4- line 97 ...conclusions. )...change to ...conclusions)
5- line 127...Cyclic...change to ...cyclic...
6-line 133...Compopund.. change to ..compound..
7-line 160..1-5, Compound..1-5, compounds..
9-line 223 put the ...2... in bold

These are the suggested typographical and stylistic corrections noted by the reviewer 1.
All the sections pointed by the reviewer 1 have been corrected. Thank you for your detailed reading.

8-In figures 3 and 5 the authors have to put the red line for compound 2.

Thank you for pointing this out. When checked in my working environment (Mac), the red line indicating the spectrum of compound 2 was shown without any problems. After the acceptance of this paper, during the proofreading and publication preparation stage, could you please check Figures 3 and 5 again at the office to confirm that the spectrum of compound 2 is shown without any problems? The same diagram is used in the graphical abstract, so please check that the red line indicating compound 2 has not disappeared.

Reviewer 2 Report

The authors describe an article entitled “Substituent Control of Near-Infrared Absorption of Triphenylamine Radical Cation”. The topic of the manuscript is interesting, and the manuscript constitutes an interesting research article concerning the development of dyes absorbing in the NIR range.

The work is well-written and a well-constructed introduction has been established by the authors. Sufficient spectra and figures are included in the manuscript for comprehension and clarity. Interesting and convincing results are also presented in this work. Overall, I think that this is a manuscript that I recommend for publication after inclusion of minor revisions.

1) What about the stability of the radical cations over time ? Could these radicals be stored for a long time ?

2) Radical cations have been prepared in this work. Besides, no EPR experiments are provided ? Why ? EPR is the best technique to characterize radicals.

3) In the chemical structure of 5, cyano groups have been introduced ? Why not to prepare the NO2 version. Considering that electron donating and electron withdrawing groups have been used, why not to use the strongest electron acceptor i.e. the nitro group ?

4) The authors mentioned at all dyes have been obtained in moderate yield. Besides, Suzuki cross-coupling reaction is an efficient reaction. Please add a paragraph explaining why the different compounds have been obtained in low yields.

Author Response

Comments from Reviewer 2  

1) What about the stability of the radical cations over time ? Could these radicals be stored for a long time ?  

Thank you for pointing this out. For compound 1, the isolation of the radical cation has been reported previously and its crystal structure has been determined (Brown, G.M.; Freeman, G.R.; Walter, R.I. Crystal structure of tri(p-biphenylyl)aminium perchlorate. J. Am. Chem. Soc. 1977, 99, doi:10.1021/ja00463a022.). Similar stability can be expected for compounds 2-4.  

2) Radical cations have been prepared in this work. Besides, no EPR experiments are provided ? Why ? EPR is the best technique to characterize radicals.  

Thank you for pointing this out. The EPR spectra of these radical cations are mentioned in the text of the first submitted manuscript and their spectra are shown in the SI (Fig. S35-38). Please see there.  

3) In the chemical structure of 5, cyano groups have been introduced ? Why not to prepare the NO2 version. Considering that electron donating and electron withdrawing groups have been used, why not to use the strongest electron acceptor i.e. the nitro group ?  

Thank you for pointing this out. As pointed out by reviewer 2, nitro groups are very strong electron-withdrawing groups. However, the introduction of nitro groups into the triphenyltriphenylamine skeleton is likely to lead to a significant decrease in solubility due to the strong intermolecular interaction. In fact, triphenylamine with three nitro groups has poor solubility in common organic solvent (Romero, I.E.; Lantaño, B.; Postigo, A.; Bonesi, S.M. Photoinduced [6π]-Electrocyclic Reaction of Mono-, Di-, and Trisubstituted Triphenylamines in Acetonitrile. A Steady-State Investigation. J. Org. Chem. 2022, doi:10.1021/acs.joc.2c00756). As an electron-withdrawing group capable of retaining good solubility in organic solvents, we have chosen cyano groups in the present study.  

4) The authors mentioned at all dyes have been obtained in moderate yield. Besides, Suzuki cross-coupling reaction is an efficient reaction. Please add a paragraph explaining why the different compounds have been obtained in low yields.  

As pointed out by reviewer 2, Suzuki coupling is a highly efficient cross-coupling. Indeed, large spots of the target compound were observed in TLC immediately after the quench of the reaction for compounds 1-5, respectively. The synthetic route in this study introduces aryl groups at three sites, so the conversion rate is expected to be moderate. In addition, recrystallization was carefully carried out to thoroughly remove impurities that might attack the radical cation. the yields shown in the SI are after these procedures. The reasons for the moderate yields are added in the synthesis section of the SI.