Electronic Absorption, Emission, and Two-Photon Absorption Properties of Some Extended 2,4,6-Triphenyl-1,3,5-Triazines

Round 1

Reviewer 1 Report

The manuscript described the synthesis, characterization, and photophysical properties of linear and nonlinear two-photon absorption characteristics of a number of 3-arm branched electron-donating fluorene chromophore molecules using a small electron-accepting core. The synthetic sequence followed the well-developed catalytic condensation reaction. Structural characterization included required various spectroscopies, high resolution mass ions, and elemental analysis. In terms of 2PA characteristics under 140 femtosecond pulse irradiation, the observed large cross-sections were determined by 2PA-excited emission fluorescence, coupled with 2PA absorption bands. The manuscript was well-prepared with DFT calculations to explain the molecular orbital properties and energy band gaps. Therefore, the manuscript can be accepted for publication at Photochem.

Author Response

This reviewer asked for no changes. We thank him for his very supportive review.

Reviewer 2 Report

The manuscript by Barnes et al. concerns a very timely subject which is the design of organic pi-conjugated molecules
with tailored photophysical properies. To that end, authors considered a series of multibranched molecules with triazine
central moiety and several substituents with varying electron-donating/-accepting capabilities.
Given the importance of the topic, clear and novel conclusions and overall high standard of academic writing, I recommend
publication after major revision.

Comments

1) In reviewer's opinion the discussion would benefit from reference to Hammett substituent constants
(see e.g. Chem. Rev. 91 (1991) 165).  This concerns location of 1PA bands maxima or 2PA cross sections.

2) Discussion of mirror-image relationships is valid provided a proper normalization is performed;
In Figure S7 authors only mention "normalized absorption / emission" on axes without any details.
In order to study mirror-image relations correctly, cross sections should be divided by 1st and 3rd power
of wavenumber in the case of absorption and emission, respectively. In the case of wavelength related units
it may be necessary to normalize by 3rd and 5th power of wavelength.
See e.g. paper of Birks & Dyson in Proc. Roy. Soc. A 275 (1963) 135.

3) It is a bit incoherent that authors jumped from CAM-B3LYP analysis of one-photon absorption spectra
to SAOP in the case of two-photon absorption spectra. In fact, no explanation is given. Authors may want
to support their choice by recent paper on this subject: J. Chem. Theory Comput. 18 (2022) 1046.

4) There are some works indicating that vibronic copupling might influence 2PA cross sections in the case
of multibranched structures (see e.g. J. Chem. Phys. 113 (2000) 7055; see also works by A. Painelli).
Such discussion would be highly relevant in the manuscript.

5) What does "Ref. AB" mean in references [75], [76] and [78]?

Author Response

CHANGES REQUESTED BY REVIEWER 2

1) In reviewer's opinion the discussion would benefit from reference to Hammett substituent constants (see e.g. Chem. Rev. 91 (1991) 165).  This concerns location of 1PA bands maxima or 2PA cross sections.

Response: We had already envisioned to investigate such relationships before submiting our article but realized that we had no sufficent data to get meaningfull correlations (5 data points for epsilon values and only 3 data points regarding 2PA cross-sections). Furthermore, in our pool of substituents, we include the NPh₂ group. As often observed, the effect of this peculiar substituent does sometimes not follow that predicted by the Hammett substituent parameter with optical data. This is most likely because its actual effect on these figures does not solely result from its electron-releasing power, but also possibly from the structural change induced by presence of two more phenyl rings which can cross-conjugate with the main pi-manifold of the compounds. Thus, rather than discussing poor Hammett-type correlations, which would have unnecessarily lengthened the manuscript without addressing significantly the actual substituent dependance of the two parameters of interest, we have prefered eluding this approach.

2) Discussion of mirror-image relationships is valid provided a proper normalization is performed; In Figure S7 authors only mention "normalized absorption / emission" on axes wihout any details. In order to study mirror-image relations correctly, cross sections should be divide by 1st and 3rd power of wavenumber in the case of absorption and emission, respectively. In the case of wavelength related units it may be necessary to normalize by 3rd and 5th power of wavelength. See e.g. paper of Birks & Dyson in Proc. Roy. Soc. A 275 (1963) 135.

Response: The reviewer is right and such normalisation are especially important when a fine vibronic structure is apparent both on absorption and emission bands. But this is not the case for our compounds. So we have presently prefered using a simpler approach recommended in many textbooks which consist in plotting the absorption and emission peaks on an abscissa scale graduated in wavenumbers. We have thus redrawn the Figure S7 accordingly in the ESI and done that for all compounds, in accordance with one of the requests of reviewer 3 (see later).

3) It is a bit incoherent that authors jumped from CAM-B3LYP analysis of one-photon absorption spectra to SAOP in the case of two-photon absorption spectra. In fact, no explanation is given. Authors may want to support their choice by recent paper on this subject: J. Chem. Theory Comput. 18 (2022) 1046.

Response: Several benchmarks of DFT functionals regarding TPA simulations have been made in the past by different authors. Presently, our choice of the functional was essentially motivated by the need to remain consistent with previous TPA calculations made on closely related systems. Furthermore, the reliability of the method of calculation for TPA spectra has been checked in one of our previous papers (see cited ref 60: N. J. Chem. 2021, 45, 15074-15081.). An excellent linear regression has been found there between the experimental and the computed TPA cross-sections in the case of series of quadrupolar oligothiophene-cored chromophores derivatives containing terminal dimesitylboryl moieties. Although the cross-sections are systematically over-estimated by this method of calculation, they follow usually the same trend as that of the experimental values, which was presently sufficient for our needs.

Note that before using the SAOP model potential recommended by Lasse Jensen and coll. (see cited ref 59: J. Chem. Theory Comput. 2016, 12, 1294-1304), we previously carried out calculations on other systems with different functionals. Among these, CAM-B3LYP was tested and gave results which were not consistent with our experimental results. In contrast, SAOP calculations gave always more consistent results, inclining us to select this functional in the framework of the TD-DFT damped cubic response theory. We have now clarified this choice in the text, stating:

« In line with previous work from us,[53, 59] the simulation of the 2PA spectra were also carried out for selected compounds using the damped cubic response theory of L. Jensen and coworkers.[60] The calculations have been performed using the SAOP functional for sake of consistency with computations already reported on these compounds (see computational details in part 4.4),[53]».

4) There are some works indicating that vibronic coupling might influence 2PA cross sections in the case of multibranched structures (see e.g. J. Chem. Phys. 113 (2000) 7055; see also works by A. Painelli). Such discussion would be highly relevant in the manuscript.

Response: We thank the reviewer for this comment and the very interesting reference given. Indeed, vibronic coupling is known to affect non-linear optical properties. In our case we were just interested by the qualitative variation of the TPA cross-sections in a homologous series of molecules and not by the accurate calculation of this property (which would have been much more time and ressouce consuming). Moreover, we expect that the vibronic coupling contribution will be of the same order of magnitude for our species that all exhibit similar three branches. As seen in the manuscript and mentioned above, our method of computation overestimates the TPA cross-sections, but gives the correct trend, which was essentially what was needed here by our experimentalists collaborators. Furthermore, the good spectral overlap between the energy of 1PA band plotted at half-wavlength and the 2PA band (ESI, Fig. S9) suggest that the overall coupling is weak in these compounds, regardless its actual origin. A sentence has now been added to clarify this to the reader in footnote [67] and the above reference is now cited as ref. [68]:

« …Note that during these computational approaches, any vibronic contribution was not envisioned,[68] but the latter should also remain weak according to the apparent energetical degeneracy of the transitions toward the A and E excited states experimentally stated (ESI, Fig. S9). »

5) What does "Ref. AB" mean in references [75], [76] and [78]?

The correct titles now indicated for these articles.

Reviewer 3 Report

The manuscript reports the study of two-photon absorption in extended 2,4,6-triphenyl-1,3,5-triazines. Even though the TPA properties of s-triazine derivatives have been extensively explored in the literature, this study continues to be of relevance due to the comparative analysis that it makes of the TPA properties of the triphenyl s-triazine derivatives with respect to isocyanurates and phenylene analogues. I recommend publication upon addressing the following suggestions.

1. The TPA properties of s-triazine derivatives have been extensively explored in the literature, yet the introduction fails to give a broad overview of the state-of-the-art being excessively focused on the triphenyl-s-triazines derivates. Introduction should give a broader overview of TPA in 1,3,5-triazine derivatives.
2. Table 1 should be organized so that the correspondence between the compound and its data is more clear. In Table 1 the data for compound 3-H should be removed. This compound was not synthesized, and it is not discussed in any way in the text.
3. In page 4, there is a reference to data in ESI showing the mirror-image relationship between absorption and emission spectra for all compounds. Such data is found in ESI figure S7 only for three of the compounds. I would suggest showing absorption and emission for all the compounds in Figure 1 of the main text by splitting this figure in several panels.
4. In page 4, it is contradictory to describe the absorption and emission spectra of all compounds as having a mirror-image relationship and at the same time refer to large structural reorganization in some compounds(Lines 123-128). Please clarify.
5. In page 5, Lines 132-133, there is a reference to solvatochromic studies but no data is shown. Is this sentence referring only to previously reported data? Where there any solvatochromic studies performed on the new compounds here synthesized? Please clarify.
6. In Table 2, the Greek letter sigma appears to be missing in some columns referring to the TPA cross-section. Please, verify.
7. In page 7, the discussion in lines 192-194 is not clear. The bathochromic shift is in absorption or emission, experimental or calculated data, and it is relative to which compound. It appears to be a comparison between 3-H and 7-H on one hand, or between 3-NPh₂ and 7-NPh₂ on the other, but the data for 3H is missing in Table 1. Please revise.
8. In page 8, Lines 204-205, that is a sentence that does not appear to make much sense. Please, revise.
9. In page 11, Lines 283-285, it is not clear to which phenomenon is this sentence referring to. Please, clarify.
10. The discussion should be revised to give a broader perspective of the relevance of the results obtained with respect to other 1,3,5-triazine derivatives discussed in the literature and not just focused on a comparative using the 1-X and 2-X compounds as a reference.
11. In section 4.3, it is mentioned that the TPA cross section was measured in solutions of 10^-4 M concentration. Usually, optical characterization is performed in solutions of 10^-6 M concentration to avoid aggregation, ensure that the Bee-Lambert law is meet and avoid inner filter effects. A comment should be made to explain why is it necessary to work at such high concentrations and what measured did the authors take to ensure the quality of the solution at such high concentrations. The concentration of the solutions used for the characterization of the linear absorption properties should also be mentioned in section 4.2.
12. The readability of the manuscript should be improved by using shorter and sharper, to the point, sentences.

Author Response

1) The TPA properties of s-triazine derivatives have been extensively explored in the literature, yet the introduction fails to give a broad overview of the state-of-the-art being excessively focused on the triphenyl-s-triazines derivates. Introduction should give a broader overview of TPA in 1,3,5-triazine derivatives

Response: In accordance with this request, we have now broadened the scope of the cited TPA investigations reported for other type of s-triazine derivatives and cited ten additionnal new references ([19]-[25] and [28]-[30]) in the introduction:

 « While extended trisalkynyl-s-triazines,[19] trisalkenyl-s-triazines[20, 21] or tris(2-thienyl)-s-triazines[22-25] have given rise to some investigations, to the best of our knowledge, only one recent theoretical paper deals specifically with the second-order NLO properties of molecules such as 3-X,[26] and only one other single paper addresses the 2PA properties of a s-triazine derivative closely related to 7-H.[27] Thus, regarding extended triphenyl-s-triazines, essentially styryl-type analogues of 3-X have been investigated so far for their 2PA properties.[28-30] »

2) Table 1 should be organized so that the correspondence between the compound and its data is more clear. In Table 1 the data for compound 3-H should be removed. This compound was not synthesized, and it is not discussed in any way in the text.

Done.

3) In page 4, there is a reference to data in ESI showing the mirror-image relationship between absorption and emission spectra for all compounds. Such data is found in ESI figure S7 only for three of the compounds. I would suggest showing absorption and emission for all the compounds in Figure 1 of the main text by splitting this figure in several panels.

Done.

4) In page 4, it is contradictory to describe the absorption and emission spectra of all compounds as having a mirror-image relationship and at the same time refer to large structural reorganization in some compounds (Lines 123-128). Please clarify.

Response: The reviewer is right and the sentences have now been re-formulated as:

« In all cases, symmetry relationships  between the first absorption and emission bands and energetical differences between their maxima (see ESI, Fig. S7) suggest that the strongly absorbing state at lowest energy is also the emitting state for all these compounds. Then, as indicated by the corresponding Stokes shifts, larger structural reorganizations and/or solvation energy changes take place in for the compounds featuring strongest electron-releasing (X = NPh₂) or electron-withdrawing (X = NO₂) substituents… »

5) In page 5, Lines 132-133, there is a reference to solvatochromic studies but no data is shown. Is this sentence referring only to previously reported data? Where there any solvatochromic studies performed on the new compounds here synthesized? Please clarify.

Response: As now precised, such data was only collected for 3-NPh₂ (see footnote [47]) and was qualitatively similar to that previously reported by Das et al. (ref. [18]).

6) In Table 2, the Greek letter sigma appears to be missing in some columns referring to the TPA cross-section. Please, verify.

Done.

7) In page 7, the discussion in lines 192-194 is not clear. The bathochromic shift is in absorption or emission, experimental or calculated data, and it is relative to which compound. It appears to be a comparison between 3-H and 7-H on one hand, or between 3-NPh₂ and 7-NPh₂ on the other, but the data for 3H is missing in Table 1. Please revise.

Response: We have now clarified this sentence:

« In line with experimental observations (Figure 1), replacing the peripheral 1,4-phenylene groups by a 2,7-fluorenyl one in 3-X when a strong electron-releasing X group such as X = NPh₂ is present leads only to a slight bathochromic shift of the first allowed absorption computed at lowest energy for 7-X (e.g. 371 nm [f = 3.03] for 3-NPh₂ vs. 375 nm [f =3.73] for 7-NPh₂, using CAM-B3LYP). In contrast, computations predict that a much more pronounced shift can be expected for a less electron-releasing substituent such as X = H (e.g. 329 nm [f = 2.46] for 3-H vs. 352 nm [f =3.23] for 7-H with the same functional), reminiscent of observations previously made between 1-X and 6-X.[33]»

8) In page 8, Lines 204-205, that is a sentence that does not appear to make much sense. Please, revise.

Response: Done. Most of the folowing section has also been slightly reformulated for clarification:

« … However, overall better results are obtained with MPW1PW91 when the transition moment are considered (ESI, Table S4).[58] The nature of the dominant excitations underlying the first absorption band (ESI, Table S4) is the same using either the CAM-B3LYP or the MPW1PW91 functional. It does not change much for all the compounds presently considered. These excitations are a pair of (nearly) degenerate transitions which would correspond to the set of degenerate transitions toward an E-type excited state under strict C_3v symmetry (i.e. an E¬A transition). In line with previous findings for 1-X and 2-X,[11, 12, 33] the first absorption band therefore corresponds to a π→π* symmetric charge-transfer (CT) between the peripheral arms and the central core.»

9) In page 11, Lines 283-285, it is not clear to which phenomenon is this sentence referring to. Please, clarify.

Response: We have now reformulated this sentence:

« Then, compared to 1-X or 2-X derivatives, the best candidate for fluorescence imaging is 3-NPh₂. This compound has indeed a two-photon brightness far above those of its known triphenylbenzene and isocyanurate analogues (1-NPh₂ and 2-NPh₂, respectively) and a 2PA peak at 820 nm, i.e. also significantly bathochromically-shifted (Table 2).[6, 9] »

10) The discussion should be revised to give a broader perspective of the relevance of the results obtained with respect to other 1,3,5-triazine derivatives discussed in the literature and not just focused on a comparative using the 1-X and 2-X compounds as a reference.

Response: In accordance with this remark, we have now added two sentences and an additional paragraph in the discussion section (as well as four additional s-triazine derivatives in Scheme 4 ; 14-17) in order to compare our results with those reported for other types of 1,3,5-triazines in the litterature. Two more references on triazines analogues have also been cited in this section ([64] and [70]):

« Actually, the value reported for 7-H is closer to that reported for compound 14 (395 GM at 790 nm), although the latter value was not recorded at the TPA peak maximum (779 nm) and possibly includes some RSA contribution.[64] »

« Comparison of the photophysical data presently measured for 3-NPh₂ with those reported for structurally related s-triazine derivatives such as 15,[19] 16[22] and 17[29] reveals larger fluorescence yields in chlorinated organic solvents for 3-NPh₂ (69% for the latter compound vs. 52%, 51% and 27%, respectively). Regarding 2PA, removing the first 1,4-phenyl ring significantly reduces the 2PA cross-section (1500 GM for 3-NPh₂ vs. 910 GM for 15), while replacing it by an 2,5-thienyl one does apparently not impact significantly this figure (1508 GM for 16) apart from slightly red-shifting the 2PA maximum (850 nm vs. 830 nm for 3-NPh₂). Then, changing the alkynyl linkers for alkenyl ones in 3-NPh₂ seems to reduce the 2PA cross-section. However care is required here since the value reported for 17 (495 GM) was obtained at a single wavelength (800 nm) which does not exactly match that of the 2PA maximum of this particular compound. »

« Such uses were already reported for structurally different s-triazine derivatives presenting much poorer figures of merit than 3-X,[70] but also for nanoparticles obtained from extended analogues of 17 and giving rise to aggregation-induced emission (AIE) in water-THF mixtures.[30] »

11) In section 4.3, it is mentioned that the TPA cross section was measured in solutions of 10^-4 M concentration. Usually, optical characterization is performed in solutions of 10^-6 M concentration to avoid aggregation, ensure that the Bee-Lambert law is meet and avoid inner filter effects. A comment should be made to explain why is it necessary to work at such high concentrations and what measured did the authors take to ensure the quality of the solution at such high concentrations. The concentration of the solutions used for the characterization of the linear absorption properties should also be mentioned in section 4.2.

Response: The two-photon excitation is a phenomenon that occurs only at the objective focal point where the photon density is high enough to allow the simultaneous absorption of two photons by the chromophore. The excitation volume is typically in the femtoliter range, so the emitted light in two-photon excited fluorescence (TPEF) is much lower than for one-photon excited fluorescence. Higher concentrations are therefore required, typically 10^-4 M. This is the usual concentration in papers dealing with TPEF measurements. It should be stressed that such concentrations are much lower than those required by other methods, such as the Z-scan technique (typically 10^-2 – 10^-3 M). Moreover, in our setup, the laser beam is focused near the cuvette window and the fluorescence is collected in epifluorescence mode. This allows to avoid the inner filter effects. We have now mentioned this in section 4.3.

However, the reviewer is right, aggregation can occur at 10^-4 M. To check this, prior to TPEF measurement, UV-visible absorption spectra are measured at this concentration in cells of 1 mm pathlength and compared with those obtained with diluted solutions in cells of 1 cm pathlength. A sentence stating that has also been added in section 4.3.

Finally, the UV-visible absorption spectra mentioned in section 4.2 have been recorded at ca. 10^-5 M. This information has also been added in the revised version of the manuscript.

12) The readability of the manuscript should be improved by using shorter and sharper, to the point, sentences.

This has now been done (whenever possible).

Reviewer 4 Report

In this work, Barnes et al. have reported one photon and two photon absorption properties of some extended 2,4,6-triphenyl-s-triazines derivatives and related analogues. The molecules of interest have been synthesized and characterized. The 2PA properties were calculated based on two-photon excited fluorescence (TPEF), and the experimental results are substantiated by DFT and TD-DFT calculations. I have enjoyed reading this paper and completely support its publication.

I just have a few minor comments and questions:

1. The damping parameters of the electronic excited states have been taken as 0.1 eV. There are several publications in the literature which also use same parameter. My question is: does this damping constant depend on the nature of the electronic excited state? This may not be relevant for this paper, but for molecules without any pi electrons, the excited states generally are of Rydberg nature and by performing standard NTO calculation, one can evaluate whether these excited orbitals are s or p or d type. Do the authors expect the damping constant to be the same in all these cases, or there is a systematic way to calculate this based on the nature of the transition?
2. The authors have talked about the conversion of 2PA cross section from atomic unit to GM unit. I strongly encourage them to include the numerical equation that connects these two units. Recent works by Steve Bradforth and coworkers, or Anna Krylov and coworkers can be consulted.
3. The authors mention that the applied average laser power arriving at the sample is typically between 0.5 and 40 mW. It seems like a rather large range. Can the authors explain the reason?

Author Response

1) The damping parameters of the electronic excited states have been taken as 0.1 eV. There are several publications in the literature which also use same parameter. My question is: does this damping constant depend on the nature of the electronic excited state? This may not be relevant for this paper, but for molecules without any pi electrons, the excited states generally are of Rydberg nature and by performing standard NTO calculation, one can evaluate whether these excited orbitals are s or p or d type. Do the authors expect the damping constant to be the same in all these cases, or there is a systematic way to calculate this based on the nature of the transition?

Response: We totally agree with the reviewer. The damping factor depends on the excited state nature. In our case we retained the recommended 0.1 eV considering that the excited state reached by the TPA is of the same nature for all compounds, because otherwise, as in the example mentoned (but presently unrelevant), the damping factor could be different. The choice of 0.1 eV has now been precised in the experimental part:

« A value of 0.1 has been considered fort he damping parameter when simulating the electronic spectra (ESI).»

2) The authors have talked about the conversion of 2PA cross section from atomic unit to GM unit. I strongly encourage them to include the numerical equation that connects these two units. Recent works by Steve Bradforth and coworkers, or Anna Krylov and coworkers can be consulted.

Response: Full details of the calculations have been given in our previous papers on related compounds (see refs. 53 or 59). Following the recommendation of this reviewer, the equation has now been recalled in the ESI to avoid lengthening the manuscript (Section 10.).

3) The authors mention that the applied average laser power arriving at the sample is typically between 0.5 and 40 mW. It seems like a rather large range. Can the authors explain the reason?

Response: The reviewer is right, this is a large range of laser power. To ensure that the fluorescence measured is only due to a two-photon absorption process without any residual one-photon absorption, we always systematically check the quadratic dependence of the fluorescence on the excitation power for all samples at every wavelength. To do that, we use several neutral filters of varying optical densities mounted on a filter wheel. This allows us to get for each compound at each data point. So, the laser power is fixed at for instance 40 mW, but thanks to the filter wheel, we can get curves of the fluorescence as a function of the excitation power. Some examples of these curves are shown in Figure S8 of the Supporting Information. To clarify this point, a sentence has now been added in section “4.3. Two-Photon Absorption Experiments”:

“The fluorescence intensity was measured at several excitation powers in this range thanks to the filter wheel. For each sample and each wavelength, the quadratic dependence of the fluorescence intensity (F) on the excitation intensity (P), i.e. the linear dependence of F on P² was systematically checked (see ESI, Fig. S8).”

Round 2

Reviewer 2 Report

Authors extensively addressed all comments. 

Reviewer 3 Report

The overall quality of the manuscript has been improved in the revised version. All the previously identified issues have been satisfactorily addressed. I recommend publication in the present form.

Reviewer 4 Report

The authors have satisfactorily answered all of my questions and have made necessary changes. I therefore, support the publication of this manuscript in its present form.